bluej tutorial (how to install and use bluej programming environment)

This page intentionally left blank

Third Edition

DataStructuresand AlgorithmAnalysis in

JavaTMTM

Boston Columbus Indianapolis New York San Francisco Upper Saddle River

Amsterdam Cape Town Dubai London Madrid Milan Munich Paris Montreal Toronto

Delhi Mexico City Sao Paulo Sydney Hong Kong Seoul Singapore Taipei Tokyo

Third Edition

DataStructuresand AlgorithmAnalysis in

JavaMark A l l e n We i ssFlorida International University

PEARSON

TM

Editorial Director: Marcia Horton Project Manager: Pat BrownEditor-in-Chief: Michael Hirsch Manufacturing Buyer: Pat BrownEditorial Assistant: Emma Snider Art Director: Jayne ConteDirector of Marketing: Patrice Jones Cover Designer: Bruce KenselaarMarketing Manager: Yezan Alayan Cover Photo: c De-Kay Dreamstime.comMarketing Coordinator: Kathryn Ferranti Media Editor: Daniel SandinDirector of Production: Vince OBrien Full-Service Project Management: IntegraManaging Editor: Jeff Holcomb Composition: IntegraProduction Project Manager: Kayla Printer/Binder: Courier Westford

Smith-Tarbox Cover Printer: Lehigh-Phoenix Color/HagerstownText Font: Berkeley-Book

Copyright c 2012, 2007, 1999 Pearson Education, Inc., publishing as Addison-Wesley. All rights reserved.Printed in the United States of America. This publication is protected by Copyright, and permission shouldbe obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or trans-mission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. To obtainpermission(s) to use material from this work, please submit a written request to Pearson Education, Inc.,Permissions Department, One Lake Street, Upper Saddle River, New Jersey 07458, or you may fax yourrequest to 201-236-3290.

Many of the designations by manufacturers and sellers to distinguish their products are claimed as trade-marks. Where those designations appear in this book, and the publisher was aware of a trademark claim, thedesignations have been printed in initial caps or all caps.

Library of Congress Cataloging-in-Publication Data

Weiss, Mark Allen.Data structures and algorithm analysis in Java / Mark Allen Weiss. 3rd ed.

p. cm.ISBN-13: 978-0-13-257627-7 (alk. paper)ISBN-10: 0-13-257627-9 (alk. paper)1. Java (Computer program language) 2. Data structures (Computer science)3. Computer algorithms. I. Title.QA76.73.J38W448 2012005.1dc23 2011035536

15 14 13 12 11CRW10 9 8 7 6 5 4 3 2 1

ISBN 10: 0-13-257627-9ISBN 13: 9780-13-257627-7

To the love of my life, Jill.

CONTENTS

Preface xvii

Chapter 1 Introduction 11.1 Whats the Book About? 1

1.2 Mathematics Review 2

1.2.1 Exponents 31.2.2 Logarithms 31.2.3 Series 41.2.4 Modular Arithmetic 51.2.5 The P Word 6

1.3 A Brief Introduction to Recursion 8

1.4 Implementing Generic Components Pre-Java 5 12

1.4.1 Using Object for Genericity 131.4.2 Wrappers for Primitive Types 141.4.3 Using Interface Types for Genericity 141.4.4 Compatibility of Array Types 16

1.5 Implementing Generic Components Using Java 5 Generics 16

1.5.1 Simple Generic Classes and Interfaces 171.5.2 Autoboxing/Unboxing 181.5.3 The Diamond Operator 181.5.4 Wildcards with Bounds 191.5.5 Generic Static Methods 201.5.6 Type Bounds 211.5.7 Type Erasure 221.5.8 Restrictions on Generics 23

vii

viii Contents

1.6 Function Objects 24

Summary 26Exercises 26References 28

Chapter 2 Algorithm Analysis 292.1 Mathematical Background 29

2.2 Model 32

2.3 What to Analyze 33

2.4 Running Time Calculations 35

2.4.1 A Simple Example 362.4.2 General Rules 362.4.3 Solutions for the Maximum Subsequence Sum Problem 392.4.4 Logarithms in the Running Time 452.4.5 A Grain of Salt 49Summary 49Exercises 50References 55

Chapter 3 Lists, Stacks, and Queues 573.1 Abstract Data Types (ADTs) 57

3.2 The List ADT 58

3.2.1 Simple Array Implementation of Lists 583.2.2 Simple Linked Lists 59

3.3 Lists in the Java Collections API 61

3.3.1 Collection Interface 613.3.2 Iterator s 613.3.3 The List Interface, ArrayList, and LinkedList 633.3.4 Example: Using remove on a LinkedList 653.3.5 ListIterators 67

3.4 Implementation of ArrayList 67

3.4.1 The Basic Class 683.4.2 The Iterator and Java Nested and Inner Classes 71

3.5 Implementation of LinkedList 75

3.6 The Stack ADT 82

3.6.1 Stack Model 82

Contents ix

3.6.2 Implementation of Stacks 833.6.3 Applications 84

3.7 The Queue ADT 92

3.7.1 Queue Model 923.7.2 Array Implementation of Queues 923.7.3 Applications of Queues 95Summary 96Exercises 96

Chapter 4 Trees 1014.1 Preliminaries 101

4.1.1 Implementation of Trees 1024.1.2 Tree Traversals with an Application 103

4.2 Binary Trees 107

4.2.1 Implementation 1084.2.2 An Example: Expression Trees 109

4.3 The Search Tree ADTBinary Search Trees 112

4.3.1 contains 1134.3.2 findMin and findMax 1154.3.3 insert 1164.3.4 remove 1184.3.5 Average-Case Analysis 120

4.4 AVL Trees 123

4.4.1 Single Rotation 1254.4.2 Double Rotation 128

4.5 Splay Trees 137

4.5.1 A Simple Idea (That Does Not Work) 1374.5.2 Splaying 139

4.6 Tree Traversals (Revisited) 145

4.7 B-Trees 147

4.8 Sets and Maps in the Standard Library 152

4.8.1 Sets 1524.8.2 Maps 1534.8.3 Implementation of TreeSet and TreeMap 1534.8.4 An Example That Uses Several Maps 154Summary 160Exercises 160References 167

x Contents

Chapter 5 Hashing 1715.1 General Idea 171

5.2 Hash Function 172

5.3 Separate Chaining 174

5.4 Hash Tables Without Linked Lists 179

5.4.1 Linear Probing 1795.4.2 Quadratic Probing 1815.4.3 Double Hashing 183

5.5 Rehashing 188

5.6 Hash Tables in the Standard Library 189

5.7 Hash Tables with Worst-Case O(1) Access 192

5.7.1 Perfect Hashing 1935.7.2 Cuckoo Hashing 1955.7.3 Hopscotch Hashing 205

5.8 Universal Hashing 211

5.9 Extendible Hashing 214


Chapter 6 Priority Queues (Heaps) 2256.1 Model 225

6.2 Simple Implementations 226

6.3 Binary Heap 226

6.3.1 Structure Property 2276.3.2 Heap-Order Property 2296.3.3 Basic Heap Operations 2296.3.4 Other Heap Operations 234

6.4 Applications of Priority Queues 238

6.4.1 The Selection Problem 2386.4.2 Event Simulation 239

6.5 d-Heaps 240

6.6 Leftist Heaps 241

6.6.1 Leftist Heap Property 2416.6.2 Leftist Heap Operations 242

6.7 Skew Heaps 249

Contents xi

6.8 Binomial Queues 252

6.8.1 Binomial Queue Structure 2526.8.2 Binomial Queue Operations 2536.8.3 Implementation of Binomial Queues 256

6.9 Priority Queues in the Standard Library 261


Chapter 7 Sorting 2717.1 Preliminaries 271

7.2 Insertion Sort 272

7.2.1 The Algorithm 2727.2.2 Analysis of Insertion Sort 272

7.3 A Lower Bound for Simple Sorting Algorithms 273

7.4 Shellsort 274

7.4.1 Worst-Case Analysis of Shellsort 2767.5 Heapsort 278

7.5.1 Analysis of Heapsort 2797.6 Mergesort 282

7.6.1 Analysis of Mergesort 2847.7 Quicksort 288

7.7.1 Picking the Pivot 2907.7.2 Partitioning Strategy 2927.7.3 Small Arrays 2947.7.4 Actual Quicksort Routines 2947.7.5 Analysis of Quicksort 2977.7.6 A Linear-Expected-Time Algorithm for Selection 300

7.8 A General Lower Bound for Sorting 302

7.8.1 Decision Trees 3027.9 Decision-Tree Lower Bounds for Selection Problems 304

7.10 Adversary Lower Bounds 307

7.11 Linear-Time Sorts: Bucket Sort and Radix Sort 310

7.12 External Sorting 315

7.12.1 Why We Need New Algorithms 3167.12.2 Model for External Sorting 3167.12.3 The Simple Algorithm 316

xii Contents

7.12.4 Multiway Merge 3177.12.5 Polyphase Merge 3187.12.6 Replacement Selection 319Summary 321Exercises 321References 327

Chapter 8 The Disjoint Set Class 3318.1 Equivalence Relations 331

8.2 The Dynamic Equivalence Problem 332

8.3 Basic Data Structure 333

8.4 Smart Union Algorithms 337

8.5 Path Compression 340

8.6 Worst Case for Union-by-Rank and Path Compression 341

8.6.1 Slowly Growing Functions 3428.6.2 An Analysis By Recursive Decomposition 3438.6.3 An O( M log *N ) Bound 3508.6.4 An O( M (M, N) ) Bound 350

8.7 An Application 352


Chapter 9 Graph Algorithms 3599.1 Denitions 359

9.1.1 Representation of Graphs 3609.2 Topological Sort 362

9.3 Shortest-Path Algorithms 366

9.3.1 Unweighted Shortest Paths 3679.3.2 Dijkstras Algorithm 3729.3.3 Graphs with Negative Edge Costs 3809.3.4 Acyclic Graphs 3809.3.5 All-Pairs Shortest Path 3849.3.6 Shortest-Path Example 384

9.4 Network Flow Problems 386

9.4.1 A Simple Maximum-Flow Algorithm 388

Contents xiii

9.5 Minimum Spanning Tree 393

9.5.1 Prims Algorithm 3949.5.2 Kruskals Algorithm 397

9.6 Applications of Depth-First Search 399

9.6.1 Undirected Graphs 4009.6.2 Biconnectivity 4029.6.3 Euler Circuits 4059.6.4 Directed Graphs 4099.6.5 Finding Strong Components 411

9.7 Introduction to NP-Completeness 412

9.7.1 Easy vs. Hard 4139.7.2 The Class NP 4149.7.3 NP-Complete Problems 415Summary 417Exercises 417References 425

Chapter 10 Algorithm DesignTechniques 429

10.1 Greedy Algorithms 429

10.1.1 A Simple Scheduling Problem 43010.1.2 Huffman Codes 43310.1.3 Approximate Bin Packing 439

10.2 Divide and Conquer 448

10.2.1 Running Time of Divide-and-Conquer Algorithms 44910.2.2 Closest-Points Problem 45110.2.3 The Selection Problem 45510.2.4 Theoretical Improvements for Arithmetic Problems 458

10.3 Dynamic Programming 462

10.3.1 Using a Table Instead of Recursion 46310.3.2 Ordering Matrix Multiplications 46610.3.3 Optimal Binary Search Tree 46910.3.4 All-Pairs Shortest Path 472

10.4 Randomized Algorithms 474

10.4.1 Random Number Generators 47610.4.2 Skip Lists 48010.4.3 Primality Testing 483

xiv Contents

10.5 Backtracking Algorithms 486

10.5.1 The Turnpike Reconstruction Problem 48710.5.2 Games 490Summary 499Exercises 499References 508

Chapter 11 Amortized Analysis 51311.1 An Unrelated Puzzle 514

11.2 Binomial Queues 514

11.3 Skew Heaps 519

11.4 Fibonacci Heaps 522

11.4.1 Cutting Nodes in Leftist Heaps 52211.4.2 Lazy Merging for Binomial Queues 52511.4.3 The Fibonacci Heap Operations 52811.4.4 Proof of the Time Bound 529

11.5 Splay Trees 531


Chapter 12 Advanced Data Structuresand Implementation 541

12.1 Top-Down Splay Trees 541

12.2 Red-Black Trees 549

12.2.1 Bottom-Up Insertion 54912.2.2 Top-Down Red-Black Trees 55112.2.3 Top-Down Deletion 556

12.3 Treaps 558

12.4 Sufx Arrays and Sufx Trees 560

12.4.1 Sufx Arrays 56112.4.2 Sufx Trees 56412.4.3 Linear-Time Construction of Sufx Arrays and Sufx Trees 567

12.5 k-d Trees 578

Contents xv

12.6 Pairing Heaps 583


Index 599

PREFACE

Purpose/GoalsThis new Java edition describes data structures, methods of organizing large amounts ofdata, and algorithm analysis, the estimation of the running time of algorithms. As computersbecome faster and faster, the need for programs that can handle large amounts of inputbecomes more acute. Paradoxically, this requires more careful attention to efciency, sinceinefciencies in programs become most obvious when input sizes are large. By analyzingan algorithm before it is actually coded, students can decide if a particular solution will befeasible. For example, in this text students look at specic problems and see how carefulimplementations can reduce the time constraint for large amounts of data from centuriesto less than a second. Therefore, no algorithm or data structure is presented without anexplanation of its running time. In some cases, minute details that affect the running timeof the implementation are explored.

Once a solution method is determined, a program must still be written. As computershave become more powerful, the problems they must solve have become larger and morecomplex, requiring development of more intricate programs. The goal of this text is to teachstudents good programming and algorithm analysis skills simultaneously so that they candevelop such programs with the maximum amount of efciency.

This book is suitable for either an advanced data structures (CS7) course or a rst-yeargraduate course in algorithm analysis. Students should have some knowledge of intermedi-ate programming, including such topics as object-based programming and recursion, andsome background in discrete math.

Summary of the Most Signicant Changes in the Third EditionThe third edition incorporates numerous bug xes, and many parts of the book haveundergone revision to increase the clarity of presentation. In addition,

Chapter 4 includes implementation of the AVL tree deletion algorithma topic oftenrequested by readers.

Chapter 5 has been extensively revised and enlarged and now contains material on twonewer algorithms: cuckoo hashing and hopscotch hashing. Additionally, a new sectionon universal hashing has been added.

Chapter 7 now contains material on radix sort, and a new section on lower boundproofs has been added. xvii

xviii Preface

Chapter 8 uses the new union/nd analysis by Seidel and Sharir, and shows theO( M(M,N) ) bound instead of the weaker O( M log N ) bound in prior editions.

Chapter 12 adds material on sufx trees and sufx arrays, including the linear-timesufx array construction algorithm by Karkkainen and Sanders (with implementation).The sections covering deterministic skip lists and AA-trees have been removed.

Throughout the text, the code has been updated to use the diamond operator fromJava 7.

ApproachAlthough the material in this text is largely language independent, programming requiresthe use of a specic language. As the title implies, we have chosen Java for this book.

Java is often examined in comparison with C++. Java offers many benets, and pro-grammers often view Java as a safer, more portable, and easier-to-use language than C++.As such, it makes a ne core language for discussing and implementing fundamental datastructures. Other important parts of Java, such as threads and its GUI, although important,are not needed in this text and thus are not discussed.

Complete versions of the data structures, in both Java and C++, are available onthe Internet. We use similar coding conventions to make the parallels between the twolanguages more evident.

OverviewChapter 1 contains review material on discrete math and recursion. I believe the only wayto be comfortable with recursion is to see good uses over and over. Therefore, recursionis prevalent in this text, with examples in every chapter except Chapter 5. Chapter 1 alsopresents material that serves as a review of inheritance in Java. Included is a discussion ofJava generics.

Chapter 2 deals with algorithm analysis. This chapter explains asymptotic analysis andits major weaknesses. Many examples are provided, including an in-depth explanation oflogarithmic running time. Simple recursive programs are analyzed by intuitively convertingthem into iterative programs. More complicated divide-and-conquer programs are intro-duced, but some of the analysis (solving recurrence relations) is implicitly delayed untilChapter 7, where it is performed in detail.

Chapter 3 covers lists, stacks, and queues. This chapter has been signicantly revisedfrom prior editions. It now includes a discussion of the Collections API ArrayListand LinkedList classes, and it provides implementations of a signicant subset of thecollections API ArrayList and LinkedList classes.

Chapter 4 covers trees, with an emphasis on search trees, including external searchtrees (B-trees). The UNIX le system and expression trees are used as examples. AVL treesand splay trees are introduced. More careful treatment of search tree implementation detailsis found in Chapter 12. Additional coverage of trees, such as le compression and gametrees, is deferred until Chapter 10. Data structures for an external medium are consideredas the nal topic in several chapters. New to this edition is a discussion of the CollectionsAPI TreeSet and TreeMap classes, including a signicant example that illustrates the use ofthree separate maps to efciently solve a problem.

Preface xix

Chapter 5 discusses hash tables, including the classic algorithms such as sepa-rate chaining and linear and quadratic probing, as well as several newer algorithms,namely cuckoo hashing and hopscotch hashing. Universal hashing is also discussed, andextendible hashing is covered at the end of the chapter.

Chapter 6 is about priority queues. Binary heaps are covered, and there is additionalmaterial on some of the theoretically interesting implementations of priority queues. TheFibonacci heap is discussed in Chapter 11, and the pairing heap is discussed in Chapter 12.

Chapter 7 covers sorting. It is very specic with respect to coding details and analysis.All the important general-purpose sorting algorithms are covered and compared. Fouralgorithms are analyzed in detail: insertion sort, Shellsort, heapsort, and quicksort. New tothis edition is radix sort and lower bound proofs for selection-related problems. Externalsorting is covered at the end of the chapter.

Chapter 8 discusses the disjoint set algorithm with proof of the running time. The anal-ysis is new. This is a short and specic chapter that can be skipped if Kruskals algorithmis not discussed.

Chapter 9 covers graph algorithms. Algorithms on graphs are interesting, not onlybecause they frequently occur in practice, but also because their running time is so heavilydependent on the proper use of data structures. Virtually all the standard algorithms arepresented along with appropriate data structures, pseudocode, and analysis of runningtime. To place these problems in a proper context, a short discussion on complexity theory(including NP-completeness and undecidability) is provided.

Chapter 10 covers algorithm design by examining common problem-solving tech-niques. This chapter is heavily fortied with examples. Pseudocode is used in these laterchapters so that the students appreciation of an example algorithm is not obscured byimplementation details.

Chapter 11 deals with amortized analysis. Three data structures from Chapters 4 and 6and the Fibonacci heap, introduced in this chapter, are analyzed.

Chapter 12 covers search tree algorithms, the sufx tree and array, the k-d tree, andthe pairing heap. This chapter departs from the rest of the text by providing complete andcareful implementations for the search trees and pairing heap. The material is structured sothat the instructor can integrate sections into discussions from other chapters. For exam-ple, the top-down red-black tree in Chapter 12 can be discussed along with AVL trees(in Chapter 4).

Chapters 19 provide enough material for most one-semester data structures courses.If time permits, then Chapter 10 can be covered. A graduate course on algorithm analysiscould cover Chapters 711. The advanced data structures analyzed in Chapter 11 can easilybe referred to in the earlier chapters. The discussion of NP-completeness in Chapter 9 isfar too brief to be used in such a course. You might nd it useful to use an additional workon NP-completeness to augment this text.

ExercisesExercises, provided at the end of each chapter, match the order in which material is pre-sented. The last exercises may address the chapter as a whole rather than a specic section.Difcult exercises are marked with an asterisk, and more challenging exercises have twoasterisks.

xx Preface

ReferencesReferences are placed at the end of each chapter. Generally the references either are his-torical, representing the original source of the material, or they represent extensions andimprovements to the results given in the text. Some references represent solutions toexercises.

SupplementsThe following supplements are available to all readers atwww.pearsonhighered.com/cssupport:

Source code for example programs

In addition, the following material is available only to qualied instructors at PearsonsInstructor Resource Center (www.pearsonhighered.com/irc). Visit the IRC or contact yourcampus Pearson representative for access.

Solutions to selected exercises Figures from the book

AcknowledgmentsMany, many people have helped me in the preparation of books in this series. Some arelisted in other versions of the book; thanks to all.

As usual, the writing process was made easier by the professionals at Pearson. Id like tothank my editor, Michael Hirsch, and production editor, Pat Brown. Id also like to thankAbinaya Rajendran and her team in Integra Software Services for their ne work puttingthe nal pieces together. My wonderful wife Jill deserves extra special thanks for everythingshe does.

Finally, Id like to thank the numerous readers who have sent e-mail messages andpointed out errors or inconsistencies in earlier versions. My World Wide Web pagewww.cis.u.edu/~weiss contains updated source code (in Java and C++), an errata list,and a link to submit bug reports.

M.A.W.Miami, Florida

C H A P T E R 1

Introduction

In this chapter, we discuss the aims and goals of this text and briey review programmingconcepts and discrete mathematics. We will

See that how a program performs for reasonably large input is just as important as itsperformance on moderate amounts of input.

Summarize the basic mathematical background needed for the rest of the book. Briey review recursion. Summarize some important features of Java that are used throughout the text.

1.1 Whats the Book About?Suppose you have a group of N numbers and would like to determine the kth largest. Thisis known as the selection problem. Most students who have had a programming courseor two would have no difculty writing a program to solve this problem. There are quite afew obvious solutions.

One way to solve this problem would be to read the N numbers into an array, sort thearray in decreasing order by some simple algorithm such as bubblesort, and then returnthe element in position k.

A somewhat better algorithm might be to read the rst k elements into an array andsort them (in decreasing order). Next, each remaining element is read one by one. As a newelement arrives, it is ignored if it is smaller than the kth element in the array. Otherwise, itis placed in its correct spot in the array, bumping one element out of the array. When thealgorithm ends, the element in the kth position is returned as the answer.

Both algorithms are simple to code, and you are encouraged to do so. The natural ques-tions, then, are which algorithm is better and, more important, is either algorithm goodenough? A simulation using a random le of 30 million elements and k = 15,000,000will show that neither algorithm nishes in a reasonable amount of time; each requiresseveral days of computer processing to terminate (albeit eventually with a correct answer).An alternative method, discussed in Chapter 7, gives a solution in about a second. Thus,although our proposed algorithms work, they cannot be considered good algorithms,because they are entirely impractical for input sizes that a third algorithm can handle in areasonable amount of time.

1

2 Chapter 1 Introduction

1 2 3 4

1 t h i s

2 w a t s

3 o a h g

4 f g d t

Figure 1.1 Sample word puzzle

A second problem is to solve a popular word puzzle. The input consists of a two-dimensional array of letters and a list of words. The object is to nd the words in the puzzle.These words may be horizontal, vertical, or diagonal in any direction. As an example, thepuzzle shown in Figure 1.1 contains the words this, two, fat, and that. The word this beginsat row 1, column 1, or (1,1), and extends to (1,4); two goes from (1,1) to (3,1); fat goesfrom (4,1) to (2,3); and that goes from (4,4) to (1,1).

Again, there are at least two straightforward algorithms that solve the problem. Foreach word in the word list, we check each ordered triple (row, column, orientation) forthe presence of the word. This amounts to lots of nested for loops but is basicallystraightforward.

Alternatively, for each ordered quadruple (row, column, orientation, number of characters)that doesnt run off an end of the puzzle, we can test whether the word indicated is in theword list. Again, this amounts to lots of nested for loops. It is possible to save some timeif the maximum number of characters in any word is known.

It is relatively easy to code up either method of solution and solve many of the real-lifepuzzles commonly published in magazines. These typically have 16 rows, 16 columns,and 40 or so words. Suppose, however, we consider the variation where only the puzzleboard is given and the word list is essentially an English dictionary. Both of the solutionsproposed require considerable time to solve this problem and therefore are not acceptable.However, it is possible, even with a large word list, to solve the problem in a matter ofseconds.

An important concept is that, in many problems, writing a working program is notgood enough. If the program is to be run on a large data set, then the running time becomesan issue. Throughout this book we will see how to estimate the running time of a programfor large inputs and, more important, how to compare the running times of two programswithout actually coding them. We will see techniques for drastically improving the speedof a program and for determining program bottlenecks. These techniques will enable us tond the section of the code on which to concentrate our optimization efforts.

1.2 Mathematics ReviewThis section lists some of the basic formulas you need to memorize or be able to deriveand reviews basic proof techniques.


1.2.1 Exponents

XAXB = XA+BXA

XB= XAB

(XA)B = XABXN + XN = 2XN = X2N2N + 2N = 2N+1

1.2.2 LogarithmsIn computer science, all logarithms are to the base 2 unless specied otherwise.

Denition 1.1.XA = B if and only if logX B = A

Several convenient equalities follow from this denition.

Theorem 1.1.

logA B =logC B

logC A; A, B,C > 0, A = 1

Proof.Let X = logC B, Y = logC A, and Z = logA B. Then, by the denition of logarithms,CX = B, CY = A, and AZ = B. Combining these three equalities yields CX = B =(CY)Z. Therefore, X = YZ, which implies Z = X/Y, proving the theorem.Theorem 1.2.

logAB = logA + log B; A, B > 0

Proof.Let X = logA, Y = log B, and Z = logAB. Then, assuming the default base of 2,2X = A, 2Y = B, and 2Z = AB. Combining the last three equalities yields 2X2Y =AB = 2Z. Therefore, X + Y = Z, which proves the theorem.Some other useful formulas, which can all be derived in a similar manner, follow.

logA/B = logA log Blog(AB) = B logA

logX < X for all X > 0

log 1 = 0, log 2 = 1, log 1,024 = 10, log 1,048,576 = 20


1.2.3 SeriesThe easiest formulas to remember are

Ni=0

2i = 2N+1 1

and the companion,

Ni=0

Ai = AN+1 1A 1

In the latter formula, if 0 < A < 1, then

Ni=0

Ai 11 A

and as N tends to , the sum approaches 1/(1 A). These are the geometric seriesformulas.

We can derive the last formula for

i=0 Ai (0 < A < 1) in the following manner. LetS be the sum. Then

S = 1 + A + A2 + A3 + A4 + A5 + Then

AS = A + A2 + A3 + A4 + A5 + If we subtract these two equations (which is permissible only for a convergent series),virtually all the terms on the right side cancel, leaving

S AS = 1which implies that

S = 11 A

We can use this same technique to compute

i=1 i/2i, a sum that occurs frequently.We write

S = 12

+ 222

+ 323

+ 424

+ 525

+

and multiply by 2, obtaining

2S = 1 + 22

+ 322

+ 423

+ 524

+ 625

+


Subtracting these two equations yields

S = 1 + 12

+ 122

+ 123

+ 124

+ 125

+ Thus, S = 2.

Another type of common series in analysis is the arithmetic series. Any such series canbe evaluated from the basic formula.

Ni=1

i = N(N + 1)2

N2

2

For instance, to nd the sum 2 + 5 + 8 + + (3k 1), rewrite it as 3(1 + 2 + 3 + +k) (1 + 1 + 1 + + 1), which is clearly 3k(k + 1)/2 k. Another way to rememberthis is to add the rst and last terms (total 3k + 1), the second and next to last terms (total3k + 1), and so on. Since there are k/2 of these pairs, the total sum is k(3k + 1)/2, whichis the same answer as before.

The next two formulas pop up now and then but are fairly uncommon.

Ni=1

i2 = N(N + 1)(2N + 1)6

N3

3

Ni=1

ik Nk+1

|k + 1| k = 1

When k = 1, the latter formula is not valid. We then need the following formula,which is used far more in computer science than in other mathematical disciplines. Thenumbers HN are known as the harmonic numbers, and the sum is known as a harmonicsum. The error in the following approximation tends to 0.57721566, which is knownas Eulers constant.

HN =N

i=1

1

i loge N

These two formulas are just general algebraic manipulations.

Ni=1

f(N) = N f(N)

Ni=n0

f(i) =N

i=1f(i)

n01i=1

f(i)

1.2.4 Modular ArithmeticWe say that A is congruent to B modulo N, written A B (mod N), if N divides A B.Intuitively, this means that the remainder is the same when either A or B is divided byN. Thus, 81 61 1 (mod 10). As with equality, if A B (mod N), then A + C B + C (mod N) and AD BD (mod N).


Often, N is a prime number. In that case, there are three important theorems.

First, if N is prime, then ab 0 (mod N) is true if and only if a 0 (mod N)or b 0 (mod N). In other words, if a prime number N divides a product of twonumbers, it divides at least one of the two numbers.

Second, if N is prime, then the equation ax 1 (mod N) has a unique solution(mod N), for all 0 < a < N. This solution 0 < x < N, is the multiplicative inverse.

Third, if N is prime, then the equation x2 a (mod N) has either two solutions(mod N), for all 0 < a < N, or no solutions.

There are many theorems that apply to modular arithmetic, and some of them requireextraordinary proofs in number theory. We will use modular arithmetic sparingly, and thepreceding theorems will sufce.

1.2.5 The P WordThe two most common ways of proving statements in data structure analysis are proofby induction and proof by contradiction (and occasionally proof by intimidation, usedby professors only). The best way of proving that a theorem is false is by exhibiting acounterexample.

Proof by InductionA proof by induction has two standard parts. The rst step is proving a base case, that is,establishing that a theorem is true for some small (usually degenerate) value(s); this step isalmost always trivial. Next, an inductive hypothesis is assumed. Generally this means thatthe theorem is assumed to be true for all cases up to some limit k. Using this assumption,the theorem is then shown to be true for the next value, which is typically k + 1. Thisproves the theorem (as long as k is nite).

As an example, we prove that the Fibonacci numbers, F0 = 1, F1 = 1, F2 = 2, F3 = 3,F4 = 5, . . . , Fi = Fi1+Fi2, satisfy Fi < (5/3)i, for i 1. (Some denitions have F0 = 0,which shifts the series.) To do this, we rst verify that the theorem is true for the trivialcases. It is easy to verify that F1 = 1 < 5/3 and F2 = 2 < 25/9; this proves the basis.We assume that the theorem is true for i = 1, 2, . . . , k; this is the inductive hypothesis. Toprove the theorem, we need to show that Fk+1 < (5/3)k+1. We have

Fk+1 = Fk + Fk1by the denition, and we can use the inductive hypothesis on the right-hand side,obtaining

Fk+1 < (5/3)k + (5/3)k1< (3/5)(5/3)k+1 + (3/5)2(5/3)k+1< (3/5)(5/3)k+1 + (9/25)(5/3)k+1


which simplies to

Fk+1 < (3/5 + 9/25)(5/3)k+1< (24/25)(5/3)k+1

< (5/3)k+1

proving the theorem.As a second example, we establish the following theorem.

Theorem 1.3.If N 1, then Ni=1 i2 = N(N+1)(2N+1)6Proof.The proof is by induction. For the basis, it is readily seen that the theorem is true whenN = 1. For the inductive hypothesis, assume that the theorem is true for 1 k N.We will establish that, under this assumption, the theorem is true for N + 1. We have

N+1i=1

i2 =N

i=1i2 + (N + 1)2

Applying the inductive hypothesis, we obtain

N+1i=1

i2 = N(N + 1)(2N + 1)6

+ (N + 1)2

= (N + 1)[

N(2N + 1)6

+ (N + 1)]

= (N + 1)2N2 + 7N + 6

6

= (N + 1)(N + 2)(2N + 3)6

Thus,

N+1i=1

i2 = (N + 1)[(N + 1) + 1][2(N + 1) + 1]6

proving the theorem.

Proof by CounterexampleThe statement Fk k2 is false. The easiest way to prove this is to compute F11=144>112.Proof by ContradictionProof by contradiction proceeds by assuming that the theorem is false and showing that thisassumption implies that some known property is false, and hence the original assumptionwas erroneous. A classic example is the proof that there is an innite number of primes. To


prove this, we assume that the theorem is false, so that there is some largest prime Pk. LetP1, P2, . . . , Pk be all the primes in order and consider

N = P1P2P3 Pk + 1Clearly, N is larger than Pk, so by assumption N is not prime. However, none ofP1, P2, . . . , Pk divides N exactly, because there will always be a remainder of 1. This isa contradiction, because every number is either prime or a product of primes. Hence, theoriginal assumption, that Pk is the largest prime, is false, which implies that the theorem istrue.

1.3 A Brief Introduction to RecursionMost mathematical functions that we are familiar with are described by a simple formula.For instance, we can convert temperatures from Fahrenheit to Celsius by applying theformula

C = 5(F 32)/9Given this formula, it is trivial to write a Java method; with declarations and bracesremoved, the one-line formula translates to one line of Java.

Mathematical functions are sometimes dened in a less standard form. As an example,we can dene a function f , valid on nonnegative integers, that satises f(0) = 0 andf(x) = 2f(x 1) + x2. From this denition we see that f(1) = 1, f(2) = 6, f(3) = 21,and f(4) = 58. A function that is dened in terms of itself is called recursive. Java allowsfunctions to be recursive.1 It is important to remember that what Java provides is merelyan attempt to follow the recursive spirit. Not all mathematically recursive functions areefciently (or correctly) implemented by Javas simulation of recursion. The idea is that therecursive function f ought to be expressible in only a few lines, just like a nonrecursivefunction. Figure 1.2 shows the recursive implementation of f .

Lines 3 and 4 handle what is known as the base case, that is, the value for whichthe function is directly known without resorting to recursion. Just as declaring f(x) =2f(x1)+ x2 is meaningless, mathematically, without including the fact that f(0) = 0, therecursive Java method doesnt make sense without a base case. Line 6 makes the recursivecall.

There are several important and possibly confusing points about recursion. A commonquestion is: Isnt this just circular logic? The answer is that although we are dening amethod in terms of itself, we are not dening a particular instance of the method in termsof itself. In other words, evaluating f(5) by computing f(5) would be circular. Evaluatingf(5) by computing f(4) is not circularunless, of course, f(4) is evaluated by eventuallycomputing f(5). The two most important issues are probably the how and why questions.

1 Using recursion for numerical calculations is usually a bad idea. We have done so to illustrate the basicpoints.


1 public static int f( int x )2 {3 if( x == 0 )4 return 0;5 else6 return 2 * f( x - 1 ) + x * x;7 }

Figure 1.2 A recursive method

In Chapter 3, the how and why issues are formally resolved. We will give an incompletedescription here.

It turns out that recursive calls are handled no differently from any others. If f is calledwith the value of 4, then line 6 requires the computation of 2 f(3) + 4 4. Thus, a callis made to compute f(3). This requires the computation of 2 f(2) + 3 3. Therefore,another call is made to compute f(2). This means that 2 f(1) + 2 2 must be evaluated.To do so, f(1) is computed as 2 f(0) + 1 1. Now, f(0) must be evaluated. Sincethis is a base case, we know a priori that f(0) = 0. This enables the completion of thecalculation for f(1), which is now seen to be 1. Then f(2), f(3), and nally f(4) can bedetermined. All the bookkeeping needed to keep track of pending calls (those started butwaiting for a recursive call to complete), along with their variables, is done by the computerautomatically. An important point, however, is that recursive calls will keep on being madeuntil a base case is reached. For instance, an attempt to evaluate f(1) will result in callsto f(2), f(3), and so on. Since this will never get to a base case, the program wontbe able to compute the answer (which is undened anyway). Occasionally, a much moresubtle error is made, which is exhibited in Figure 1.3. The error in Figure 1.3 is thatbad(1) is dened, by line 6, to be bad(1). Obviously, this doesnt give any clue as to whatbad(1) actually is. The computer will thus repeatedly make calls to bad(1) in an attemptto resolve its values. Eventually, its bookkeeping system will run out of space, and theprogram will terminate abnormally. Generally, we would say that this method doesnt workfor one special case but is correct otherwise. This isnt true here, since bad(2) calls bad(1).Thus, bad(2) cannot be evaluated either. Furthermore, bad(3), bad(4), and bad(5) all makecalls to bad(2). Since bad(2) is unevaluable, none of these values are either. In fact, this

1 public static int bad( int n )2 {3 if( n == 0 )4 return 0;5 else6 return bad( n / 3 + 1 ) + n - 1;7 }

Figure 1.3 A nonterminating recursive method


program doesnt work for any nonnegative value of n, except 0. With recursive programs,there is no such thing as a special case.

These considerations lead to the rst two fundamental rules of recursion:

1. Base cases. You must always have some base cases, which can be solved withoutrecursion.

2. Making progress. For the cases that are to be solved recursively, the recursive call mustalways be to a case that makes progress toward a base case.

Throughout this book, we will use recursion to solve problems. As an example ofa nonmathematical use, consider a large dictionary. Words in dictionaries are dened interms of other words. When we look up a word, we might not always understand thedenition, so we might have to look up words in the denition. Likewise, we might notunderstand some of those, so we might have to continue this search for a while. Becausethe dictionary is nite, eventually either (1) we will come to a point where we understandall of the words in some denition (and thus understand that denition and retrace ourpath through the other denitions) or (2) we will nd that the denitions are circularand we are stuck, or that some word we need to understand for a denition is not in thedictionary.

Our recursive strategy to understand words is as follows: If we know the meaning of aword, then we are done; otherwise, we look the word up in the dictionary. If we understandall the words in the denition, we are done; otherwise, we gure out what the denitionmeans by recursively looking up the words we dont know. This procedure will terminateif the dictionary is well dened but can loop indenitely if a word is either not dened orcircularly dened.

Printing Out NumbersSuppose we have a positive integer, n, that we wish to print out. Our routine will have theheading printOut(n). Assume that the only I/O routines available will take a single-digitnumber and output it to the terminal. We will call this routine printDigit; for example,printDigit(4) will output a 4 to the terminal.

Recursion provides a very clean solution to this problem. To print out 76234, we needto rst print out 7623 and then print out 4. The second step is easily accomplished withthe statement printDigit(n%10), but the rst doesnt seem any simpler than the originalproblem. Indeed it is virtually the same problem, so we can solve it recursively with thestatement printOut(n/10).

This tells us how to solve the general problem, but we still need to make sure thatthe program doesnt loop indenitely. Since we havent dened a base case yet, it is clearthat we still have something to do. Our base case will be printDigit(n) if 0 n < 10.Now printOut(n) is dened for every positive number from 0 to 9, and larger numbers aredened in terms of a smaller positive number. Thus, there is no cycle. The entire methodis shown in Figure 1.4.


1 public static void printOut( int n ) /* Print nonnegative n */2 {3 if( n >= 10 )4 printOut( n / 10 );5 printDigit( n % 10 );6 }

Figure 1.4 Recursive routine to print an integer

We have made no effort to do this efciently. We could have avoided using the modroutine (which can be very expensive) because n%10 = n n/10 10.2

Recursion and InductionLet us prove (somewhat) rigorously that the recursive number-printing program works. Todo so, well use a proof by induction.

Theorem 1.4.The recursive number-printing algorithm is correct for n 0.Proof (by induction on the number of digits in n).First, if n has one digit, then the program is trivially correct, since it merely makesa call to printDigit. Assume then that printOut works for all numbers of k or fewerdigits. A number of k + 1 digits is expressed by its rst k digits followed by its leastsignicant digit. But the number formed by the rst k digits is exactly n/10, which,by the inductive hypothesis, is correctly printed, and the last digit is n mod 10, sothe program prints out any (k + 1)-digit number correctly. Thus, by induction, allnumbers are correctly printed.

This proof probably seems a little strange in that it is virtually identical to the algorithmdescription. It illustrates that in designing a recursive program, all smaller instances of thesame problem (which are on the path to a base case) may be assumed to work correctly. Therecursive program needs only to combine solutions to smaller problems, which are mag-ically obtained by recursion, into a solution for the current problem. The mathematicaljustication for this is proof by induction. This gives the third rule of recursion:

3. Design rule. Assume that all the recursive calls work.

This rule is important because it means that when designing recursive programs, you gen-erally dont need to know the details of the bookkeeping arrangements, and you dont haveto try to trace through the myriad of recursive calls. Frequently, it is extremely difcultto track down the actual sequence of recursive calls. Of course, in many cases this is anindication of a good use of recursion, since the computer is being allowed to work out thecomplicated details.

2 x is the largest integer that is less than or equal to x.


The main problem with recursion is the hidden bookkeeping costs. Although thesecosts are almost always justiable, because recursive programs not only simplify the algo-rithm design but also tend to give cleaner code, recursion should never be used as asubstitute for a simple for loop. Well discuss the overhead involved in recursion in moredetail in Section 3.6.

When writing recursive routines, it is crucial to keep in mind the four basic rules ofrecursion:

1. Base cases. You must always have some base cases, which can be solved withoutrecursion.

2. Making progress. For the cases that are to be solved recursively, the recursive call mustalways be to a case that makes progress toward a base case.

3. Design rule. Assume that all the recursive calls work.

4. Compound interest rule. Never duplicate work by solving the same instance of a problemin separate recursive calls.

The fourth rule, which will be justied (along with its nickname) in later sections, is thereason that it is generally a bad idea to use recursion to evaluate simple mathematical func-tions, such as the Fibonacci numbers. As long as you keep these rules in mind, recursiveprogramming should be straightforward.

1.4 Implementing Generic ComponentsPre-Java 5

An important goal of object-oriented programming is the support of code reuse. An impor-tant mechanism that supports this goal is the generic mechanism: If the implementationis identical except for the basic type of the object, a generic implementation can be usedto describe the basic functionality. For instance, a method can be written to sort an arrayof items; the logic is independent of the types of objects being sorted, so a generic methodcould be used.

Unlike many of the newer languages (such as C++, which uses templates to implementgeneric programming), before version 1.5, Java did not support generic implementationsdirectly. Instead, generic programming was implemented using the basic concepts of inher-itance. This section describes how generic methods and classes can be implemented in Javausing the basic principles of inheritance.

Direct support for generic methods and classes was announced by Sun in June 2001 asa future language addition. Finally, in late 2004, Java 5 was released and provided supportfor generic methods and classes. However, using generic classes requires an understandingof the pre-Java 5 idioms for generic programming. As a result, an understanding of howinheritance is used to implement generic programs is essential, even in Java 5.


1.4.1 Using Object for GenericityThe basic idea in Java is that we can implement a generic class by using an appropriatesuperclass, such as Object. An example is the MemoryCell class shown in Figure 1.5.

There are two details that must be considered when we use this strategy. The rst isillustrated in Figure 1.6, which depicts a main that writes a "37" to a MemoryCell object andthen reads from the MemoryCell object. To access a specic method of the object, we mustdowncast to the correct type. (Of course, in this example, we do not need the downcast,since we are simply invoking the toString method at line 9, and this can be done for anyobject.)

A second important detail is that primitive types cannot be used. Only referencetypes are compatible with Object. A standard workaround to this problem is discussedmomentarily.

1 // MemoryCell class2 // Object read( ) --> Returns the stored value3 // void write( Object x ) --> x is stored45 public class MemoryCell6 {7 // Public methods8 public Object read( ) { return storedValue; }9 public void write( Object x ) { storedValue = x; }

1011 // Private internal data representation12 private Object storedValue;13 }

Figure 1.5 A generic MemoryCell class (pre-Java 5)

1 public class TestMemoryCell2 {3 public static void main( String [ ] args )4 {5 MemoryCell m = new MemoryCell( );67 m.write( "37" );8 String val = (String) m.read( );9 System.out.println( "Contents are: " + val );

10 }11 }

Figure 1.6 Using the generic MemoryCell class (pre-Java 5)


1.4.2 Wrappers for Primitive TypesWhen we implement algorithms, often we run into a language typing problem: We havean object of one type, but the language syntax requires an object of a different type.

This technique illustrates the basic theme of a wrapper class. One typical use is tostore a primitive type, and add operations that the primitive type either does not supportor does not support correctly.

In Java, we have already seen that although every reference type is compatible withObject, the eight primitive types are not. As a result, Java provides a wrapper class for eachof the eight primitive types. For instance, the wrapper for the int type is Integer. Eachwrapper object is immutable (meaning its state can never change), stores one primitivevalue that is set when the object is constructed, and provides a method to retrieve thevalue. The wrapper classes also contain a host of static utility methods.

As an example, Figure 1.7 shows how we can use the MemoryCell to store integers.

1.4.3 Using Interface Types for GenericityUsing Object as a generic type works only if the operations that are being performed canbe expressed using only methods available in the Object class.

Consider, for example, the problem of nding the maximum item in an array of items.The basic code is type-independent, but it does require the ability to compare any twoobjects and decide which is larger and which is smaller. Thus we cannot simply nd themaximum of an array of Objectwe need more information. The simplest idea would be tond the maximum of an array of Comparable. To determine order, we can use the compareTomethod that we know must be available for all Comparables. The code to do this is shownin Figure 1.8, which provides a main that nds the maximum in an array of String or Shape.

It is important to mention a few caveats. First, only objects that implement theComparable interface can be passed as elements of the Comparable array. Objects that have acompareTo method but do not declare that they implement Comparable are not Comparable,and do not have the requisite IS-A relationship. Thus, it is presumed that Shape implements

1 public class WrapperDemo2 {3 public static void main( String [ ] args )4 {5 MemoryCell m = new MemoryCell( );67 m.write( new Integer( 37 ) );8 Integer wrapperVal = (Integer) m.read( );9 int val = wrapperVal.intValue( );

10 System.out.println( "Contents are: " + val );11 }12 }

Figure 1.7 An illustration of the Integer wrapper class


1 class FindMaxDemo2 {3 /**4 * Return max item in arr.5 * Precondition: arr.length > 06 */7 public static Comparable findMax( Comparable [ ] arr )8 {9 int maxIndex = 0;

1011 for( int i = 1; i < arr.length; i++ )12 if( arr[ i ].compareTo( arr[ maxIndex ] ) > 0 )13 maxIndex = i;1415 return arr[ maxIndex ];16 }1718 /**19 * Test findMax on Shape and String objects.20 */21 public static void main( String [ ] args )22 {23 Shape [ ] sh1 = { new Circle( 2.0 ),24 new Square( 3.0 ),25 new Rectangle( 3.0, 4.0 ) };2627 String [ ] st1 = { "Joe", "Bob", "Bill", "Zeke" };2829 System.out.println( findMax( sh1 ) );30 System.out.println( findMax( st1 ) );31 }32 }

Figure 1.8 A generic findMax routine, with demo using shapes and strings (pre-Java 5)

the Comparable interface, perhaps comparing areas of Shapes. It is also implicit in the testprogram that Circle, Square, and Rectangle are subclasses of Shape.

Second, if the Comparable array were to have two objects that are incompatible (e.g., aString and a Shape), the compareTo method would throw a ClassCastException. This is theexpected (indeed, required) behavior.

Third, as before, primitives cannot be passed as Comparables, but the wrappers workbecause they implement the Comparable interface.

Fourth, it is not required that the interface be a standard library interface.Finally, this solution does not always work, because it might be impossible to declare

that a class implements a needed interface. For instance, the class might be a library class,


while the interface is a user-dened interface. And if the class is nal, we cant extend itto create a new class. Section 1.6 offers another solution for this problem, which is thefunction object. The function object uses interfaces also and is perhaps one of the centralthemes encountered in the Java library.

1.4.4 Compatibility of Array TypesOne of the difculties in language design is how to handle inheritance for aggregate types.Suppose that Employee IS-A Person. Does this imply that Employee[] IS-A Person[]? In otherwords, if a routine is written to accept Person[] as a parameter, can we pass an Employee[]as an argument?

At rst glance, this seems like a no-brainer, and Employee[] should be type-compatiblewith Person[]. However, this issue is trickier than it seems. Suppose that in addition toEmployee, Student IS-A Person. Suppose the Employee[] is type-compatible with Person[].Then consider this sequence of assignments:

Person[] arr = new Employee[ 5 ]; // compiles: arrays are compatiblearr[ 0 ] = new Student( ... ); // compiles: Student IS-A Person

Both assignments compile, yet arr[0] is actually referencing an Employee, and StudentIS-NOT-A Employee. Thus we have type confusion. The runtime system cannot throw aClassCastException since there is no cast.

The easiest way to avoid this problem is to specify that the arrays are not type-compatible. However, in Java the arrays are type-compatible. This is known as a covariantarray type. Each array keeps track of the type of object it is allowed to store. Ifan incompatible type is inserted into the array, the Virtual Machine will throw anArrayStoreException.

The covariance of arrays was needed in earlier versions of Java because otherwise thecalls on lines 29 and 30 in Figure 1.8 would not compile.

1.5 Implementing Generic ComponentsUsing Java 5 Generics

Java 5 supports generic classes that are very easy to use. However, writing generic classesrequires a little more work. In this section, we illustrate the basics of how generic classesand methods are written. We do not attempt to cover all the constructs of the language,which are quite complex and sometimes tricky. Instead, we show the syntax and idiomsthat are used throughout this book.


1.5.1 Simple Generic Classes and InterfacesFigure 1.9 shows a generic version of the MemoryCell class previously depicted in Figure 1.5.Here, we have changed the name to GenericMemoryCell because neither class is in a packageand thus the names cannot be the same.

When a generic class is specied, the class declaration includes one or more typeparameters enclosed in angle brackets after the class name. Line 1 shows that theGenericMemoryCell takes one type parameter. In this instance, there are no explicit restric-tions on the type parameter, so the user can create types such as GenericMemoryCelland GenericMemoryCell but not GenericMemoryCell. Inside the GenericMemo-ryCell class declaration, we can declare elds of the generic type and methods that usethe generic type as a parameter or return type. For example, in line 5 of Figure 1.9, thewrite method for GenericMemoryCell requires a parameter of type String. Passinganything else will generate a compiler error.

Interfaces can also be declared as generic. For example, prior to Java 5 the Comparableinterface was not generic, and its compareTo method took an Object as the parameter. Asa result, any reference variable passed to the compareTo method would compile, even ifthe variable was not a sensible type, and only at runtime would the error be reported asa ClassCastException. In Java 5, the Comparable class is generic, as shown in Figure 1.10.The String class, for instance, now implements Comparable and has a compareTomethod that takes a String as a parameter. By making the class generic, many of the errorsthat were previously only reported at runtime become compile-time errors.

1 public class GenericMemoryCell2 {3 public AnyType read( )4 { return storedValue; }5 public void write( AnyType x )6 { storedValue = x; }78 private AnyType storedValue;9 }

Figure 1.9 Generic implementation of the MemoryCell class

1 package java.lang;23 public interface Comparable4 {5 public int compareTo( AnyType other );6 }

Figure 1.10 Comparable interface, Java 5 version which is generic


1.5.2 Autoboxing/UnboxingThe code in Figure 1.7 is annoying to write because using the wrapper class requirescreation of an Integer object prior to the call to write, and then the extraction of the intvalue from the Integer, using the intValue method. Prior to Java 5, this is required becauseif an int is passed in a place where an Integer object is required, the compiler will generatean error message, and if the result of an Integer object is assigned to an int, the compilerwill generate an error message. This resulting code in Figure 1.7 accurately reects thedistinction between primitive types and reference types, yet it does not cleanly express theprogrammers intent of storing ints in the collection.

Java 5 recties this situation. If an int is passed in a place where an Integer isrequired, the compiler will insert a call to the Integer constructor behind the scenes. Thisis known as autoboxing. And if an Integer is passed in a place where an int is required,the compiler will insert a call to the intValue method behind the scenes. This is knownas auto-unboxing. Similar behavior occurs for the seven other primitive/wrapper pairs.Figure 1.11a illustrates the use of autoboxing and unboxing in Java 5. Note that the enti-ties referenced in the GenericMemoryCell are still Integer objects; int cannot be substitutedfor Integer in the GenericMemoryCell instantiations.

1.5.3 The Diamond OperatorIn Figure 1.11a, line 5 is annoying because since m is of type GenericMemoryCell,it is obvious that object being created must also be GenericMemoryCell; any othertype parameter would generate a compiler error. Java 7 adds a new language feature, knownas the diamond operator, that allows line 5 to be rewritten as

GenericMemoryCell m = new GenericMemoryCell( );

The diamond operator simplies the code, with no cost to the developer, and we use itthroughout the text. Figure 1.11b shows the Java 7 version, incorporating the diamondoperator.

1 class BoxingDemo2 {3 public static void main( String [ ] args )4 {5 GenericMemoryCell m = new GenericMemoryCell( );67 m.write( 37 );8 int val = m.read( );9 System.out.println( "Contents are: " + val );

10 }11 }

Figure 1.11a Autoboxing and unboxing (Java 5)


1 class BoxingDemo2 {3 public static void main( String [ ] args )4 {5 GenericMemoryCell m = new GenericMemoryCell( );67 m.write( 5 );8 int val = m.read( );9 System.out.println( "Contents are: " + val );

10 }11 }

Figure 1.11b Autoboxing and unboxing (Java 7, using diamond operator)

1.5.4 Wildcards with BoundsFigure 1.12 shows a static method that computes the total area in an array of Shapes (weassume Shape is a class with an area method; Circle and Square extend Shape). Suppose wewant to rewrite the method so that it works with a parameter that is Collection.Collection is described in Chapter 3; for now, the only important thing about it is that itstores a collection of items that can be accessed with an enhanced for loop. Because ofthe enhanced for loop, the code should be identical, and the resulting code is shown inFigure 1.13. If we pass a Collection, the code works. However, what happens ifwe pass a Collection? The answer depends on whether a Collection IS-ACollection. Recall from Section 1.4.4 that the technical term for this is whether wehave covariance.

In Java, as we mentioned in Section 1.4.4, arrays are covariant. So Square[] IS-AShape[]. On the one hand, consistency would suggest that if arrays are covariant, thencollections should be covariant too. On the other hand, as we saw in Section 1.4.4, thecovariance of arrays leads to code that compiles but then generates a runtime exception(an ArrayStoreException). Because the entire reason to have generics is to generate compiler

1 public static double totalArea( Shape [ ] arr )2 {3 double total = 0;45 for( Shape s : arr )6 if( s != null )7 total += s.area( );89 return total;

10 }

Figure 1.12 totalArea method for Shape[]


1 public static double totalArea( Collection arr )2 {3 double total = 0;45 for( Shape s : arr )6 if( s != null )7 total += s.area( );89 return total;

10 }

Figure 1.13 totalArea method that does not work if passed a Collection

1 public static double totalArea( Collection


1 public static boolean contains( AnyType [ ] arr, AnyType x )2 {3 for( AnyType val : arr )4 if( x.equals( val ) )5 return true;67 return false;8 }

Figure 1.15 Generic static method to search an array

class declaration. Sometimes the specic type is important perhaps because one of thefollowing reasons apply:

1. The type is used as the return type.

2. The type is used in more than one parameter type.

3. The type is used to declare a local variable.

If so, then an explicit generic method with type parameters must be declared.For instance, Figure 1.15 illustrates a generic static method that performs a sequential

search for value x in array arr. By using a generic method instead of a nongeneric methodthat uses Object as the parameter types, we can get compile-time errors if searching for anApple in an array of Shapes.

The generic method looks much like the generic class in that the type parameter listuses the same syntax. The type parameters in a generic method precede the return type.

1.5.6 Type BoundsSuppose we want to write a findMax routine. Consider the code in Figure 1.16. This codecannot work because the compiler cannot prove that the call to compareTo at line 6 is valid;compareTo is guaranteed to exist only if AnyType is Comparable. We can solve this problem

1 public static AnyType findMax( AnyType [ ] arr )2 {3 int maxIndex = 0;45 for( int i = 1; i < arr.length; i++ )6 if( arr[ i ].compareTo( arr[ maxIndex ] ) > 0 )7 maxIndex = i;89 return arr[ maxIndex ];

10 }

Figure 1.16 Generic static method to nd largest element in an array that does not work


1 public static


to generic methods that have an erased return type, casts are inserted automatically. If ageneric class is used without a type parameter, the raw class is used.

One important consequence of type erasure is that the generated code is not muchdifferent than the code that programmers have been writing before generics and in fact isnot any faster. The signicant benet is that the programmer does not have to place castsin the code, and the compiler will do signicant type checking.

1.5.8 Restrictions on GenericsThere are numerous restrictions on generic types. Every one of the restrictions listed hereis required because of type erasure.

Primitive TypesPrimitive types cannot be used for a type parameter. Thus GenericMemoryCell is illegal.You must use wrapper classes.

instanceof testsinstanceof tests and typecasts work only with raw type. In the following code

GenericMemoryCell cell1 = new GenericMemoryCell( );cell1.write( 4 );Object cell = cell1;GenericMemoryCell cell2 = (GenericMemoryCell) cell;String s = cell2.read( );

the typecast succeeds at runtime since all types are GenericMemoryCell. Eventually, a run-time error results at the last line because the call to read tries to return a String but cannot.As a result, the typecast will generate a warning, and a corresponding instanceof test isillegal.

Static ContextsIn a generic class, static methods and elds cannot refer to the classs type variables since,after erasure, there are no type variables. Further, since there is really only one raw class,static elds are shared among the classs generic instantiations.

Instantiation of Generic TypesIt is illegal to create an instance of a generic type. If T is a type variable, the statement

T obj = new T( ); // Right-hand side is illegal

is illegal. T is replaced by its bounds, which could be Object (or even an abstract class), sothe call to new cannot make sense.

Generic Array ObjectsIt is illegal to create an array of a generic type. If T is a type variable, the statement

T [ ] arr = new T[ 10 ]; // Right-hand side is illegal


is illegal. T would be replaced by its bounds, which would probably be Object, and then thecast (generated by type erasure) to T[] would fail because Object[] IS-NOT-A T[]. Becausewe cannot create arrays of generic objects, generally we must create an array of the erasedtype and then use a typecast. This typecast will generate a compiler warning about anunchecked type conversion.

Arrays of Parameterized TypesInstantiation of arrays of parameterized types is illegal. Consider the following code:

1 GenericMemoryCell [ ] arr1 = new GenericMemoryCell[ 10 ];2 GenericMemoryCell cell = new GenericMemoryCell( ); cell.write( 4.5 );3 Object [ ] arr2 = arr1;4 arr2[ 0 ] = cell;5 String s = arr1[ 0 ].read( );

Normally, we would expect that the assignment at line 4, which has the wrong type,would generate an ArrayStoreException. However, after type erasure, the array type isGenericMemoryCell[], and the object added to the array is GenericMemoryCell, so there isno ArrayStoreException. Thus, this code has no casts, yet it will eventually generate aClassCastException at line 5, which is exactly the situation that generics are supposed toavoid.

1.6 Function ObjectsIn Section 1.5, we showed how to write generic algorithms. As an example, the genericmethod in Figure 1.16 can be used to nd the maximum item in an array.

However, that generic method has an important limitation: It works only for objectsthat implement the Comparable interface, using compareTo as the basis for all comparisondecisions. In many situations, this approach is not feasible. For instance, it is a stretchto presume that a Rectangle class will implement Comparable, and even if it does, thecompareTo method that it has might not be the one we want. For instance, given a 2-by-10rectangle and a 5-by-5 rectangle, which is the larger rectangle? The answer would dependon whether we are using area or width to decide. Or perhaps if we are trying to t the rect-angle through an opening, the larger rectangle is the rectangle with the larger minimumdimension. As a second example, if we wanted to nd the maximum string (alphabeti-cally last) in an array of strings, the default compareTo does not ignore case distinctions, soZEBRA would be considered to precede alligator alphabetically, which is probably notwhat we want.

The solution in these situations is to rewrite findMax to accept two parameters: an arrayof objects and a comparison function that explains how to decide which of two objects isthe larger and which is the smaller. In effect, the objects no longer know how to comparethemselves; instead, this information is completely decoupled from the objects in the array.

An ingenious way to pass functions as parameters is to notice that an object containsboth data and methods, so we can dene a class with no data and one method and pass

1.6 Function Objects 25

1 // Generic findMax, with a function object.2 // Precondition: a.size( ) > 0.3 public static 4 AnyType findMax( AnyType [ ] arr, Comparator


1 package java.util;23 public interface Comparator4 {5 int compare( AnyType lhs, AnyType rhs );6 }

Figure 1.19 The Comparator interface

implements Comparator. This object is of type CaseInsensitiveCompare, which is aclass we write.

In Chapter 4, we will give an example of a class that needs to order the items it stores.We will write most of the code using Comparable and show the adjustments needed to usethe function objects. Elsewhere in the book, we will avoid the detail of function objects tokeep the code as simple as possible, knowing that it is not difcult to add function objectslater.

Summary

This chapter sets the stage for the rest of the book. The time taken by an algorithm con-fronted with large amounts of input will be an important criterion for deciding if it is agood algorithm. (Of course, correctness is most important.) Speed is relative. What is fastfor one problem on one machine might be slow for another problem or a different machine.We will begin to address these issues in the next chapter and will use the mathematicsdiscussed here to establish a formal model.

Exercises

1.1 Write a program to solve the selection problem. Let k = N/2. Draw a table showingthe running time of your program for various values of N.

1.2 Write a program to solve the word puzzle problem.

1.3 Write a method to output an arbitrary double number (which might be negative)using only printDigit for I/O.

1.4 C allows statements of the form

#include lename

which reads lename and inserts its contents in place of the include statement.Include statements may be nested; in other words, the le lename may itself con-tain an include statement, but, obviously, a le cant include itself in any chain.Write a program that reads in a le and outputs the le as modied by the includestatements.

Exercises 27

1.5 Write a recursive method that returns the number of 1s in the binary representationof N. Use the fact that this is equal to the number of 1s in the representation of N/2,plus 1, if N is odd.

1.6 Write the routines with the following declarations:

public void permute( String str );private void permute( char [ ] str, int low, int high );

The rst routine is a driver that calls the second and prints all the permutations ofthe characters in String str. If str is "abc", then the strings that are output are abc,acb, bac, bca, cab, and cba. Use recursion for the second routine.

1.7 Prove the following formulas:a. logX < X for all X > 0b. log(AB) = B logA

1.8 Evaluate the following sums:a.

i=0 14ib.

i=0 i4ic.

i=0 i

2

4id.

i=0 i

N

4i

1.9 Estimate

Ni=N/2

1

i

1.10 What is 2100 (mod 5)?

1.11 Let Fi be the Fibonacci numbers as dened in Section 1.2. Prove the following:a.N2

i=1 Fi = FN 2b. FN < N, with = (1 +

5)/2

c. Give a precise closed-form expression for FN.

1.12 Prove the following formulas:a.N

i=1(2i 1) = N2b.N

i=1i3 =(N

i=1i)2

1.13 Design a generic class, Collection, that stores a collection of Objects (in an array),along with the current size of the collection. Provide public methods isEmpty,makeEmpty, insert, remove, and isPresent. isPresent(x) returns true if and only ifan Object that is equal to x (as dened by equals) is present in the collection.

1.14 Design a generic class, OrderedCollection, that stores a collection of Comparables(in an array), along with the current size of the collection. Provide public methodsisEmpty, makeEmpty, insert, remove, findMin, and findMax. findMin and findMax returnreferences to the smallest and largest, respectively, Comparable in the collection (ornull if the collection is empty).


1.15 Dene a Rectangle class that provides getLength and getWidth methods. Using thefindMax routines in Figure 1.18, write a main that creates an array of Rectangleand nds the largest Rectangle rst on the basis of area, and then on the basis ofperimeter.

References

There are many good textbooks covering the mathematics reviewed in this chapter. A smallsubset is [1], [2], [3], [11], [13], and [14]. Reference [11] is specically geared toward theanalysis of algorithms. It is the rst volume of a three-volume series that will be citedthroughout this text. More advanced material is covered in [8].

Throughout this book we will assume a knowledge of Java [4], [6], [7]. The materialin this chapter is meant to serve as an overview of the features that we will use in thistext. We also assume familiarity with recursion (the recursion summary in this chapter ismeant to be a quick review). We will attempt to provide hints on its use where appropriatethroughout the textbook. Readers not familiar with recursion should consult [14] or anygood intermediate programming textbook.

General programming style is discussed in several books. Some of the classics are [5],[9], and [10].

1. M. O. Albertson and J. P. Hutchinson, Discrete Mathematics with Algorithms, John Wiley &Sons, New York, 1988.

2. Z. Bavel, Math Companion for Computer Science, Reston Publishing Co., Reston, Va., 1982.3. R. A. Brualdi, Introductory Combinatorics, North-Holland, New York, 1977.4. G. Cornell and C. S. Horstmann, Core Java, Vol. I, 8th ed., Prentice Hall, Upper Saddle

River, N.J., 2009.5. E. W. Dijkstra, A Discipline of Programming, Prentice Hall, Englewood Cliffs, N.J., 1976.6. D. Flanagan, Java in a Nutshell, 5th ed., OReilly and Associates, Sebastopol, Calif., 2005.7. J. Gosling, B. Joy, G. Steele, and G. Bracha, The Java Language Specication, 3d ed., Addison-

Wesley, Reading, Mass., 2005.8. R. L. Graham, D. E. Knuth, and O. Patashnik, Concrete Mathematics, Addison-Wesley,

Reading, Mass., 1989.9. D. Gries, The Science of Programming, Springer-Verlag, New York, 1981.

10. B. W. Kernighan and P. J. Plauger, The Elements of Programming Style, 2d ed., McGraw-Hill,New York, 1978.

11. D. E. Knuth, The Art of Computer Programming, Vol. 1: Fundamental Algorithms, 3d ed.,Addison-Wesley, Reading, Mass., 1997.

12. F. S. Roberts, Applied Combinatorics, Prentice Hall, Englewood Cliffs, N.J., 1984.13. A. Tucker, Applied Combinatorics, 2d ed., John Wiley & Sons, New York, 1984.14. M. A. Weiss, Data Structures and Problem Solving Using Java, 4th ed., Addison-Wesley,

Boston, Mass., 2010.

C H A P T E R 2

Algorithm Analysis

An algorithm is a clearly specied set of simple instructions to be followed to solve aproblem. Once an algorithm is given for a problem and decided (somehow) to be correct,an important step is to determine how much in the way of resources, such as time or space,the algorithm will require. An algorithm that solves a problem but requires a year is hardlyof any use. Likewise, an algorithm that requires hundreds of gigabytes of main memory isnot (currently) useful on most machines.

In this chapter, we shall discuss

How to estimate the time required for a program. How to reduce the running time of a program from days or years to fractions of asecond.

The results of careless use of recursion. Very efcient algorithms to raise a number to a power and to compute the greatestcommon divisor of two numbers.

2.1 Mathematical BackgroundThe analysis required to estimate the resource use of an algorithm is generally a theoreticalissue, and therefore a formal framework is required. We begin with some mathematicaldenitions.

Throughout the book we will use the following four denitions:

Denition 2.1.T(N) = O(f(N)) if there are positive constants c and n0 such that T(N) cf(N) whenN n0.Denition 2.2.T(N) = (g(N)) if there are positive constants c and n0 such that T(N) cg(N) whenN n0.Denition 2.3.T(N) = (h(N)) if and only if T(N) = O(h(N)) and T(N) = (h(N)).

29

30 Chapter 2 Algorithm Analysis

Denition 2.4.T(N) = o(p(N)) if for all positive constants c there exists an n0 such that T(N) < cp(N)when N > n0. Less formally, T(N) = o(p(N)) if T(N) = O(p(N)) and T(N) = (p(N)).The idea of these denitions is to establish a relative order among functions. Given two

functions, there are usually points where one function is smaller than the other function,so it does not make sense to claim, for instance, f(N) < g(N). Thus, we compare theirrelative rates of growth. When we apply this to the analysis of algorithms, we shall seewhy this is the important measure.

Although 1,000N is larger than N2 for small values of N, N2 grows at a faster rate, andthus N2 will eventually be the larger function. The turning point is N = 1,000 in this case.The rst denition says that eventually there is some point n0 past which c f(N) is alwaysat least as large as T(N), so that if constant factors are ignored, f(N) is at least as big asT(N). In our case, we have T(N) = 1,000N, f(N) = N2, n0 = 1,000, and c = 1. We couldalso use n0 = 10 and c = 100. Thus, we can say that 1,000N = O(N2) (order N-squared).This notation is known as Big-Oh notation. Frequently, instead of saying order . . . , onesays Big-Oh . . . .

If we use the traditional inequality operators to compare growth rates, then therst denition says that the growth rate of T(N) is less than or equal to () that off(N). The second denition, T(N) = (g(N)) (pronounced omega), says that thegrowth rate of T(N) is greater than or equal to () that of g(N). The third denition,T(N) = (h(N)) (pronounced theta), says that the growth rate of T(N) equals (=)the growth rate of h(N). The last denition, T(N) = o(p(N)) (pronounced little-oh),says that the growth rate of T(N) is less than (

2.1 Mathematical Background 31

Function Name

c Constant

logN Logarithmic

log2 N Log-squared

N Linear

N logN

N2 Quadratic

N3 Cubic

2N Exponential

Figure 2.1 Typical growth rates

The important things to know are

Rule 1.If T1(N) = O(f(N)) and T2(N) = O(g(N)), then

(a) T1(N) + T2(N) = O(f(N) + g(N)) (intuitively and less formally it isO(max(f(N), g(N))) ),

(b) T1(N) T2(N) = O(f(N) g(N)).Rule 2.If T(N) is a polynomial of degree k, then T(N) = (Nk).Rule 3.logk N = O(N) for any constant k. This tells us that logarithms grow very slowly.This information is sufcient to arrange most of the common functions by growth rate

(see Figure 2.1).Several points are in order. First, it is very bad style to include constants or low-order

terms inside a Big-Oh. Do not say T(N) = O(2N2) or T(N) = O(N2+N). In both cases, thecorrect form is T(N) = O(N2). This means that in any analysis that will require a Big-Ohanswer, all sorts of shortcuts are possible. Lower-order terms can generally be ignored, andconstants can be thrown away. Considerably less precision is required in these cases.

Second, we can always determine the relative growth rates of two functions f(N) andg(N) by computing limN f(N)/g(N), using LHpitals rule if necessary.1 The limit canhave four possible values:

The limit is 0: This means that f(N) = o(g(N)). The limit is c = 0: This means that f(N) = (g(N)).

1 LHpitals rule states that if limN f(N) = and limN g(N) = , then limN f(N)/g(N) =limN f (N)/g(N), where f (N) and g(N) are the derivatives of f(N) and g(N), respectively.


The limit is : This means that g(N) = o(f(N)). The limit does not exist: There is no relation (this will not happen in our context).

Using this method almost always amounts to overkill. Usually the relation betweenf(N) and g(N) can be derived by simple algebra. For instance, if f(N) = N logN andg(N) = N1.5, then to decide which of f(N) and g(N) grows faster, one really needs todetermine which of logN and N0.5 grows faster. This is like determining which of log2 Nor N grows faster. This is a simple problem, because it is already known that N grows fasterthan any power of a log. Thus, g(N) grows faster than f(N).

One stylistic note: It is bad to say f(N) O(g(N)), because the inequality is implied bythe denition. It is wrong to write f(N) O(g(N)), which does not make sense.

As an example of the typical kinds of analysis that are performed, consider the problemof downloading a le over the Internet. Suppose there is an initial 3-sec delay (to set upa connection), after which the download proceeds at 1.5 M(bytes)/sec. Then it followsthat if the le is N megabytes, the time to download is described by the formula T(N) =N/1.5 + 3. This is a linear function. Notice that the time to download a 1,500M le(1,003 sec) is approximately (but not exactly) twice the time to download a 750M le (503sec). This is typical of a linear function. Notice, also, that if the speed of the connectiondoubles, both times decrease, but the 1,500M le still takes approximately twice the timeto download as a 750M le. This is the typical characteristic of linear-time algorithms, andit is why we write T(N) = O(N), ignoring constant factors. (Although using Big-Thetawould be more precise, Big-Oh answers are typically given.)

Observe, too, that this behavior is not true of all algorithms. For the rst selectionalgorithm described in Section 1.1, the running time is controlled by the time it takes toperform a sort. For a simple sorting algorithm, such as the suggested bubble sort, when theamount of input doubles, the running time increases by a factor of four for large amountsof input. This is because those algorithms are not linear. Instead, as we will see when wediscuss sorting, trivial sorting algorithms are O(N2), or quadratic.

2.2 ModelIn order to analyze algorithms in a formal framework, we need a model of computation.Our model is basically a normal computer, in which instructions are executed sequentially.Our model has the standard repertoire of simple instructions, such as addition, multipli-cation, comparison, and assignment, but, unlike the case with real computers, it takesexactly one time unit to do anything (simple). To be reasonable, we will assume that, likea modern computer, our model has xed-size (say, 32-bit) integers and that there are nofancy operations, such as matrix inversion or sorting, that clearly cannot be done in onetime unit. We also assume innite memory.

This model clearly has some weaknesses. Obviously, in real life, not all operations takeexactly the same time. In particular, in our model one disk read counts the same as anaddition, even though the addition is typically several orders of magnitude faster. Also, byassuming innite memory, we ignore the fact that the cost of a memory access can increasewhen slower memory is used due to larger memory requirements.

2.3 What to Analyze 33

2.3 What to AnalyzeThe most important resource to analyze is generally the running time. Several factorsaffect the running time of a program. Some, such as the compiler and computer used,are obviously beyond the scope of any theoretical model, so, although they are important,we cannot deal with them here. The other main factors are the algorithm used and theinput to the algorithm.

Typically, the size of the input is the main consideration. We dene two functions,Tavg(N) and Tworst(N), as the average and worst-case running time, respectively, used byan algorithm on input of size N. Clearly, Tavg(N) Tworst(N). If there is more than oneinput, these functions may have more than one argument.

Occasionally the best-case performance of an algorithm is analyzed. However, this isoften of little interest, because it does not represent typical behavior. Average-case perfor-mance often reects typical behavior, while worst-case performance represents a guaranteefor performance on any possible input. Notice, also, that, although in this chapter we ana-lyze Java code, these bounds are really bounds for the algorithms rather than programs.Programs are an implementation of the algorithm in a particular programming language,and almost always the details of the programming language do not affect a Big-Oh answer. Ifa program is running much more slowly than the algorithm analysis suggests, there may bean implementation inefciency. This is more common in languages (like C++) where arrayscan be inadvertently copied in their entirety, instead of passed with references. However,this can occur in Java, too. Thus in future chapters we will analyze the algorithms ratherthan the programs.

Generally, the quantity required is the worst-case time, unless otherwise specied. Onereason for this is that it provides a bound for all input, including particularly bad input,which an average-case analysis does not provide. The other reason is that average-casebounds are usually much more difcult to compute. In some instances, the denitionof average can affect the result. (For instance, what is average input for the followingproblem?)

As an example, in the next section, we shall consider the following problem:

Maximum Subsequence Sum Problem.

Given (possibly negative) integers A1,A2, . . . ,AN, nd the maximum value ofj

k=i Ak.(For convenience, the maximum subsequence sum is 0 if all the integers are negative.)Example:

For input 2, 11, 4, 13, 5, 2, the answer is 20 (A2 through A4).This problem is interesting mainly because there are so many algorithms to solve it,

and the performance of these algorithms varies drastically. We will discuss four algo-rithms to solve this problem. The running time on some computer (the exact computeris unimportant) for these algorithms is given in Figure 2.2.

There are several important things worth noting in this table. For a small amount ofinput, the algorithms all run in a blink of the eye, so if only a small amount of input isexpected, it might be silly to expend a great deal of effort to design a clever algorithm.On the other hand, there is a large market these days for rewriting programs that werewritten ve years ago based on a no-longer-valid assumption of small input size. These


Algorithm Time

Input 1 2 3 4

Size O(N3) O(N2) O(N logN) O(N)

N = 100 0.000159 0.000006 0.000005 0.000002N = 1,000 0.095857 0.000371 0.000060 0.000022N = 10,000 86.67 0.033322 0.000619 0.000222N = 100,000 NA 3.33 0.006700 0.002205N = 1,000,000 NA NA 0.074870 0.022711

Figure 2.2 Running times of several algorithms for maximumsubsequence sum (in seconds)

programs are now too slow, because they used poor algorithms. For large amounts ofinput, algorithm 4 is clearly the best choice (although algorithm 3 is still usable).

Second, the times given do not include the time required to read the input. For algo-rithm 4, the time merely to read in the input from a disk is likely to be an order ofmagnitude larger than the time required to solve the problem. This is typical of manyefcient algorithms. Reading the data is generally the bottleneck; once the data are read,the problem can be solved quickly. For inefcient algorithms this is not true, and signicantcomputer resources must be used. Thus it is important that, whenever possible, algorithmsbe efcient enough not to be the bottleneck of a problem.

Notice that algorithm 4, which is linear, exhibits the nice behavior that as the prob-lem size increases by a factor of ten, the running time also increases by a factor of ten.

0

Run

ning

Tim

e

10 20 30 40 50 60 70 80 90 100Input Size (N)

LinearO(N log N )

QuadraticCubic

Figure 2.3 Plot (N vs. time) of various algorithms

2.4 Running Time Calculations 35

00

Run

ning

Tim

e

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Input Size (N)

LinearO(N log N )

QuadraticCubic

Figure 2.4 Plot (N vs. time) of various algorithms

Algorithm 2, which is quadratic, does not have this behavior; a tenfold increase in inputsize yields roughly a hundredfold (102) increase in running time. And algorithm 1, whichis cubic, yields a thousandfold (103) increase in running time. We would expect algorithm1 to take nearly 9,000 seconds (or two and half hours) to complete for N = 100,000.Similarly, we would e

bluej tutorial (how to install and use bluej programming environment)

Documents

generic components

java mark allen weiss

java computer program

marcia horton project

simple generic classes

primitive types

interface types

generic static method