LECTURE 27:DEQUESCSC 212 – Data Structures

Roses are red and violets are blue

Implement push, pop, & top

And you’re a Stack too!

Stack & ADT Memory Aid

public interface Stack<E> {public E top() throws EmptyStackException;

public E pop() throws EmptyStackException;

public void push(E elem);

public int size();public boolean isEmpty();

}

Stack Interface

Queue Memory Aid

It’s hard writing rhymes with

enqueue, dequeue, and

front

public interface Queue<E> {public E front() throws EmptyQueueException;

public E dequeue() throws EmptyQueueException;

public void enqueue(E elem);

public int size();public boolean isEmpty();

}

Queue ADT

Stack vs. Queue

Access data with Stack in LIFO order LLast IIn-FFirst OOut is totally unfair (unless

always late) Data accessed in Queue using FIFO

order FFirst IIn-FFirst OOut ensures early bird gets

the worm

Order read if

Stack

Cannot access both sides of Collection Transplant waiting lists Help center phone banks Grandpa dealing cards for money

Stack only works on one end All insertions & removals from the Stack’s

top Queue limits how each side used

Elements can only be added to end Front allows accessing & removal of

elements

Still Have Limits

Pronounced “deck” (like on a house) Mnemonic for DDouble EEnded QUEQUEue dequeue ≠ deque and do not sound alike

Structure that provides access to both ends Combines Stack & Queue concepts Is also able to add elements to start

Deque ADT

public interface Deque<E> {public E getFirst() /* front() */ throws EmptyDequeException;public E removeFirst() /* dequeue() */ throws EmptyDequeException;

public E getLast() /* top() */ throws EmptyDequeException;public E removeLast() /* pop() */ throws EmptyDequeException;

public void addFirst(E elem); /* Brand new! */public void addLast(E elem); /* push() & enqueue() */

public int size();public boolean isEmpty();

}

Deque Interface

VIP Lines & Lucky Losers

Class defines fields aliased to first & last nodes Doubly-linked list enables O(1) time for all

methods Add elements by adding new Node at end Update sentinels next/previous to remove

element

Linked-list based Deque

head rear

Ø

retVal

Ø

DEQUES, like QUEUES, have both ends move addFirst & removeFirst moves its front Structure’s end moved by addLast & removeLast

Ends of a array-based DEQUE like clock time Identical to Queue and how it works,

except… …occasionally need to subtract from index,

also

Circular Access

q

rf r r r r rr r f

Array-based DEQUE Operations Uses property of clock math

Remainder of result is all that matters But the values sign is also important -1 % 4 == -1, for example

To get this to work, use a cheap trick Adding size of the array does not change

result

(-1 + 6) % 6 (3 + 6) % 6= 5 % 6 = 9 % 6= 5 = 3

Start week #10 assignment Due by 5PM next Tuesday

Continue programming assignment #3 Messages are not always sent to everyone!

Read section 6.1 in book before class What if we wanted to access all the items? Could we get something like a resizable

array? How do ranks differ from indices?

Before Next Lecture…