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ICS 491: Competitve Programming – Lecture 1: Introduction

ICS 491: Competitive ProgrammingProf. Nodari Sitchinava

www.algoparc.ics.hawaii.edu

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Lecture 1: Introduction


Competitive Programming?

Goal:

Learn to solve computational problems quickly and efficiently


Competitive Programming?

Goal:

Learn to solve computational problems quickly and efficiently

Improve problem solving skillsPrepare for ICPC programming competitionTechnical skill of a Computer Science major

Why bother?

ICPC website: https://icpc.baylor.edu


UH Manoa ICPC Performance

2015

2017


UH Manoa 2017 ICPC Performance


Course Info

Course Website:http://www2.hawaii.edu/~nodari/teaching/f18/

Instructor: Nodari SitchinavaEmail: [email protected] (Put “ICS 491” in the Subject)Office: POST 309COffice Hours: Wednesdays 3-4pm

TA: Branden OgataEmail: [email protected] (Put “ICS 491” in the Subject)Office: POST 314-6Office Hours: Tuesday 2-3pm and Thursday 3-4pm


Course Info


Course Info

Prerequisites:‘B’ or better in ICS 311Fluency in Java, C or C++


Course Info

Prerequisites:‘B’ or better in ICS 311Fluency in Java, C or C++

Recommended Reading:Steven Halim and Felix Halim: Competitive Programming 3, 3rdEdition, Lulu Press, 2014. [CP3]Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest andClifford Stein, Introduction to Algorithms, 3rd Edition, The MITPress, 2009 [CLRS]


Course Info (cont.)

Grading:

70%: Weekly mini-contests (75 min)15%: Midterm contest (150 min)15%: Final contest (120 min)


Course Info (cont.)

Grading:


Cheating will result in an immediate F


Course Info (cont.)

Practice problems on UVa Online Judge:

Assigned weekly, but not gradedPreparation for weekly mini-contestsNeed an account on https://uva.onlinejudge.org

Grading:


Cheating will result in an immediate F


Programming Environment

OS: Ubuntu 16.04 with GNOME (on USB)Languages:

Java 8 (OpenJDK 1.8)C (gcc v5.4 with gnu11 extensions)C++ (g++ v5.4 with gnu++14 extensions)

2017-2018 ICPC environment without Python/Kotlin

Contests submissions:DOM Judge server (similar to UVa Online Judge)

JDK JavaDocsC++ STL docs

No internet access!


ICPC Regional Competition

On Saturday, November 3, 2018 in Laie, HIEligibility criteriahttps://icpc.baylor.edu/regionals/rules

If eligible and interestedTell me (via email) by the end of Sept 30, 2018

Potential team selection competition on

Saturday, Oct 20, 2018



Boot from the provided USB:Insert USBPress power button




username:password:

Your home directory will be wiped clean at each login




username:password:

Open Firefox and visit https://judge.ics.hawaii.edu/domjudge/

Your home directory will be wiped clean at each login

Pick username wisely:Competition ranking by usernameWill be public information


Familiarize yourself with the environment

E.g.: write a program that prints ”Hello World!” and exits

Compile using one of the following:

C:gcc -g -O2 -std=gnu11 -static ${files} -lm

C++:g++ -g -O2 -std=gnu++14 -static ${files}Java:javac -encoding UTF-8 -sourcepath . -d . ${files}

And run/test your program

For Java, run it with:java -Dfile.encoding=UTF-8 -XX:+UseSerialGC -Xss64m

-Xms1920m -Xmx1920m ${file}


DomJudge Responses

Accepted (AC)

Presentation Error (PE)Wrong Answer (WA)Time Limit Exceeded (TLE)Memory Limit Exceeded (MLE)Runtime Error (RTE)

Good:

Bad:


Sample Contest – 45 min

Compiling:

C:gcc -g -O2 -std=gnu11 -static ${files} -lm

C++:g++ -g -O2 -std=gnu++14 -static ${files}Java:javac -encoding UTF-8 -sourcepath . -d . ${files}

java -Dfile.encoding=UTF-8 -XX:+UseSerialGC -Xss64m

-Xms1920m -Xmx1920m ${file}

Running Java:


Contest


Problem A: TopK

Solutions:


Problem A: TopK

Solutions:

Scan K times, looking for smallest and removing it


Problem A: TopK

Solutions:


O(K · N)


Problem A: TopK

Solutions:


O(K · N)

Sort, report first K


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )

Build a heap, extract min K times


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)

Partition around K -th smallest element, sort first K elements


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)

Partition around K -th smallest element, sort first K elementsO(N + K log K )


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)


Use algorithm::partial_sort(A, A+K, A+N)O(N + K log K )


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)


Use algorithm::partial_sort(A, A+K, A+N)

O(N log K )

O(N + K log K )


Problem A: TopK

Solutions:


O(K · N)


O(N log N + K )


O(N + K log N)


Use algorithm::partial_sort(A, A+K, A+N)

O(N log K )

O(N + K log K )

Which one is better?


Problem A: TopK, N ≤ 1M, K ≤ 100

n Worst-case AC Algorithm≤ [10..11] O(n!), O(n6)≤ [15..18] O(2n · n2)≤ [18..22] O(2n · n)≤ 100 O(n4)≤ 400 O(n3)≤ 2K O(n2 log2 n)≤ 10K O(n2)≤ 1M O(n log2 n)

≤ 100M O(n)

Modern processors: ≈ 100M = 108 ops per second




≤ 100M O(n)


Use the simplest algorithm within the time budget




≤ 100M O(n)


Use the simplest algorithm within the time budget

O(N log K ) ≈ 7M finishes within 1 second


Problem B: Delayed Work

Understand the problem




cost(M) = (K/M) · P + X ·MFind M that minimizes cost(M)





Take a derivative and set to 0cost ′(M) = −K P/M2 + X = 0






Solve for M⇒ K P/M2 = X⇒ M =

√K P/X







√K P/X

Return cost(M) = K P√K P/X

+ X√

K P/X =√

K PX +√

XK P







√K P/X


+ X√

K P/X =√

K PX +√

XK P

WrongAnswer (WA)!







√K P/X


+ X√

K P/X =√

K PX +√

XK P

WrongAnswer (WA)!

M must be integer







√K P/X

Return min {cost(bMc), cost(dMe)}

= min{

K PbMc

+ XbMc, K PdMe

+ XdMe}

M must be integer







√K P/X

Return min {cost(bMc), cost(dMe)}

= min{

K PbMc

+ XbMc, K PdMe

+ XdMe}

Don’t forget to format the output

M must be integer


Problem C: CardSorting

Card games are common in contestsThe problem is a simplest exerciseLearn to process strange inputsMap to integers and sort them


Problem C: CardSorting

Card games are common in contestsThe problem is a simplest exerciseLearn to process strange inputsMap to integers and sort them

x = 13 · suit + (value − 2)

suit intSpades 0Hearts 1Diamonds 2Clubs 3

face value2-9 2-9T (10) 10J (Jack) 11Q (Queen) 12K (King) 13A (Ace) 14


Problem D: OddPalindrom, |s| ≤ 100

Solution 1:



Solution 1:

Check every substring if it’s an even palindrom

Analysis



Solution 1:

Check every substring if it’s an even palindrom

O(|s|2) substringsO(|s|) time to check if a palindrom

O(|s|3) time

s3 = 1M ⇒ < 1second

Analysis



Solution 2:



Solution 2:

Claim: If the string is not odd, then it contains a 2-characterpalindrom



Solution 2:


Much simpler solution:Scan the string and check if there are two identicalcharacters next to each otherOutput ”Not odd.” the moment they are found



Solution 2:



Much easier (and faster) to code



Solution 2:



Analysis:

A single scan: O(|s|)

Much easier (and faster) to code


Problem types

Class will cover many common problem typesMost rely on ICS 311 algorithms/data structures for efficientimplementations

Used as subroutinesDon’t need proofsOften library implementations existMust know asymptotic runtimes


Lessons learned

Understand the problem firstThe first solution might be too tedious to implementUse paperDraw examples

Do a quick runtime analysisOnly start implementing if close to passing RTE

Test programDon’t rely on sample testcasesFind difficult testcases.Try large testcases

Master Programming Language(s)Don’t waste time reading documentation

Practice, practice, practice...

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