Queue, Deque, and Priority Queue Implementations Chapter 11 Copyright ©2012 by Pearson Education,...

Queue, Deque, and Priority Queue Implementations

Chapter 11

Copyright ©2012 by Pearson Education, Inc. All rights reserved

Contents

• A Linked Implementation of a Queue

• An Array-Based Implementation of a Queue A Circular Array A Circular Array with One Unused Location

• A Vector-Based Implementation of a Queue


Contents

• Circular Linked Implementations of a Queue A Two-Part Circular Linked Chain

• Java Class Library: The Class AbstractQueue

• A Doubly Linked Implementation of a Deque

• Possible Implementations of a Priority Queue


Objectives

• Implement ADT queue by using chain of linked nodes, an array, or vector

• Add or delete nodes at either end of chain of doubly linked nodes

• Implement ADT deque by using chain of doubly linked nodes

• Implement ADT priority queue by using array or chain of linked nodes


Linked Implementation of a Queue

• Consider chain of linked nodes Head reference insufficient Must also have tail reference

• Which should be front of queue? Head easier to be front of queue for entry

removal Adding entries at tail/back of queue easily

done


Figure 11-1 A chain of linked nodes that implements a queue


Linked Implementation of a Queue

• Note code for linked Implementation Listing 11-1


Figure 11-2 (a) Before adding a new node to an empty chain; (b) after adding it

Note: Code listing filesmust be in same folder

as PowerPoint filesfor links to work

Note: Code listing filesmust be in same folder

as PowerPoint filesfor links to work

Figure 11-3 (a) Before, (b) during, and (c) after adding a new node to the end of a nonempty chain that has a tail reference


FIGURE 11-4 (a) A queue of more than one entry; (b) after removing the entry at the front of the queue


Figure 11-5 (a) A queue of one entry; (b) after removing the entry at the front of the queue


Array-Based Implementation of a Queue

• Listing 11-2 : Array named queue, queue[0] is front frontIndex, backIndex are indices of front

and back of queue

• Now to decide … With queue[0] always as front, must shift

elements Instead, move frontIndex Then we run off the end of the array!?


Figure 11-6 An array that represents a queue without moving any entries: (a) initially;

(b) after removing the entry at the front twice;Copyright ©2012 by Pearson Education, Inc. All rights reserved

Figure 11-6 An array that represents a queue without moving any entries: (c) after several more additions and removals;

(d) after two additions that wrap around to the beginning of the array;


Array-Based Implementation of a Queue

• Solution: “Circular” array First location follows last Increment pointers with modulo operator

backIndex = (backIndex + 1) % queue.length;frontIndex = (frontIndex + 1) % queue.length;


Figure 11-6 An array that represents a queue without moving any entries: (a) initially; (b) after removing

the entry at the front twice;Copyright ©2012 by Pearson Education, Inc. All rights reserved

Figure 11-6 An array that represents a queue without moving any entries: (c) after several more additions and removals;

(d) after two additions that wrap around to the beginning of the arrayCopyright ©2012 by Pearson Education, Inc. All rights reserved

Figure 11-7 A circular array that represents a queue: (a) when full; (b) after removing two entries;


Figure 11-7 A circular array that represents a queue: (c) after removing three more entries; (d) after removing all but one entry;

(e) after removing the remaining entryCopyright ©2012 by Pearson Education, Inc. All rights reserved

Circular Array with One Unused Element

• Allows detection of empty vs. full queue Examine frontIndex, backIndex


Figure 11-8 A seven-location circular array that contains at most six entries of a queue





Figure 11-9 An array-based queue: (a) initially; (b) after removing its front entry by incrementing frontIndex;


Figure 11-9 An array-based queue(c) after removingits front entry by setting queue[frontIndex] to null and then

incrementing frontIndexCopyright ©2012 by Pearson Education, Inc. All rights reserved

Vector-Based Implementation of a Queue

• Front of queue at beginning of vector

• Vector add method used at back of queue

• Remove from front of queue Vector takes care of moving elements No indices needed

• Vector manages additional space as needed

• View Listing 11-3


Figure 11-11 A vector that represents a queue


Circular Linked Implementations of a Queue


Figure 11-12 A circular linked chain with an external reference to its last node that (a) has more than one node;

(b) has one node; (c) is empty

Two-Part Circular Linked Chain


Figure 11-13 A two-part circular linked chain that represents both a queue and the nodes available to the queue

Figure 11-14 A two-part circular linked chain that represents a queue: (a) when it is empty; (b) after adding one entry;


Figure 11-14 A two-part circular linked chain that represents a queue: (c) after adding three more entries;


Figure 11-14 A two-part circular linked chain that represents a queue: (d) after removing the front entry;


Figure 11-14 A two-part circular linked chain that represents a queue: (e) after adding one more entry


Two-Part Circular Linked Chain

• Outline of the class, Listing 11-4


Figure 11-15 A chain that requires a new node for an addition to a queue: (a) before the addition;

Figure 11-15 A chain that requires a new node for an addition to a queue: (b) after the addition


Figure 11-16 (a) A chain with nodes available for additions to a queue;


Figure 11-16 (b) the chain after one addition;


Figure 11-16 (c) the chain after another addition;


Java Class Library: The Class AbstractQueue

• Methods provided public boolean add(T newEntry) public boolean offer(T newEntry) public T remove() public T poll() public T element() public T peek() public boolean isEmpty() public void clear() public int size()


Doubly Linked Implementation of a Deque

• Problem: each node in a chain references only the next node. We need to be able to move backward from a

node

• This suggests a doubly linked chain


Doubly Linked Implementation of a Deque

• View implementation, Listing 11-5


Figure 11-17 A doubly linked chain with head, tail references

Figure 11-18 Adding to the back of a nonempty deque: (a) after the new node is allocated;

(b) after the addition is completeCopyright ©2012 by Pearson Education, Inc. All rights reserved

Figure 11-19 (a) A deque containing at least two entries; (b) after removing the first node and obtaining a reference to the

deque’s new first entryCopyright ©2012 by Pearson Education, Inc. All rights reserved

Possible Implementations of a Priority Queue


Figure 11-20 Two possible implementations of a priority queue using (a) an array; (b) a chain of linked nodes

End

Chapter 11


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