Stacks and Queues

RHS – SWC 2

Stacks

• A stack is an abstract data structure, with some special properties:– Insertion and deletion is only allowed at the

end of the stack (usually called the top)– Due to this restriction, elements are removed

from the stack in the opposite order in which they were added

– This is called LIFO order (Last In, First Out)

RHS – SWC 3

Stacks

public interface Stack<T>

{

void push(T element);

T pop();

T peek();

}

RHS – SWC 4

Stacks

A

B

C

D

push(A)

push(B)

push(C)

push(D)

A = pop()

B = pop()

C = pop()

D = pop()

RHS – SWC 5

Stacks

• Note that we have not stated exactly how a Stack is implemented

• Can be done in various ways (array, linked list, etc.)

• However, data structure should support efficient (O(1)) insertion and removal at the end of the collection

RHS – SWC 6

Stacks

• What are stacks used for…?– Expression evaluation– Memory management– …any situation where LIFO order is the

proper order for processing data

RHS – SWC 7

Stacks

RHS – SWC 8

Stacks

• The (in)famous HP15-C pocket calculator used Reverse Polish Notation for entering and evaluating expressions

• This notation is very closely related to stacks

• Loved by many, hated by more…

RHS – SWC 9

Stacks

• 12 x 37 is not typed in as:

• but rather as:

1 2 x 3 7 =

1 2 E 3 7 xE

RHS – SWC 10

Stacks

12

37

X

Push(12)

Push(37)

Push(X)

12 = pop()

37 = pop()

X = pop()

Push(12 x 37)

444

RHS – SWC 11

Queues

• A queue is (also) an abstract data structure, with some special properties:– Insertion is only allowed at the end of the

queue (usually called the tail)– Deletion is only allowed at the start of the

queue (usually called the head)– Elements are removed from the stack in the

same order in which they were added– This is called FIFO order (First In, First Out)

RHS – SWC 12

Queues

public interface Queue<T>

{

void add(T element);

T remove();

T peek();

}

RHS – SWC 13

Queues

A

B

C

D

add(A)

add(B)

add(C)

add(D)

A = remove()

B = remove()

C = remove()

D = remove()

RHS – SWC 14

Queue

• Like for a Stack, a Queue can be implemented in various ways

• Data structure should support efficient (O(1)) insertion and removal at the start and end of the collection

• Linked list likely choice

RHS – SWC 15

Queue

• What are queues used for…?– Event handling– All situations where we need to process data

in the order it is produced, but cannot process data immediately

RHS – SWC 16

Stacks and Queues in Java

• Stack<T> is part of Java library, and can be used as-is

• Queue<T> is an interface!

• There are several implementations of the Queue interface availabe

• See the documentation for details