Queues and Linked Lists

Queues and Linked Lists

• Queues
• Linked Lists
• Double-Ended Queues

Queues

• A queue differs from a stack in that its insertion and removal routines follows the first-in-first-out (FIFO) principle.
• Elements may be inserted at any time, but only the element which has been in the queue the longest may be removed.
• Elements are inserted at the rear (enqueued) and removed from the front (dequeued)

The Queue Abstract Data Type

• The queue has two fundamental methods:
• enqueue(o): Insert object o at the rear of the queue
• dequeue(): Remove the object from the front of the queue and return it; an error occurs if the queue is empty
• These support methods should also be defined:
• size(): Return the number of objects in the queue
• isEmpty(): Return a boolean value that indicates whether the queue is empty
• front(): Return, but do not remove, the front object in the queue; an error occurs if the queue is empty

An Array-Based Queue

• Create a queue using an array in a circular fashion
• A maximum size N is specified, e.g. N = 1,000.
• The queue consists of an N-element array Q and two integer variables:
• -f, index of the front element
• -r, index of the element after the rear one
• “normal configuration”

• Questions:
• What does f=r mean?
• How do we compute the number of elements in the queue from f and r?

An Array-Based Queue (contd.)

• Pseudo-Code (cont'd.)

• Algorithm size():
• return (N - f + r) mod N
• Algorithm isEmpty():
• return (f = r)
• Algorithm front():
• if isEmpty() then
• throw a QueueEmptyException
• return Q[f]

• Algorithm dequeue():
• if isEmpty() then
• throw a QueueEmptyException
• temp  Q[f]
• Q[f]  null
• f  (f + 1) mod N
• return temp
• Algorithm enqueue(o):
• if size = N - 1 then
• throw a QueueFullException
• Q[r]  o

Implementing a Queue with a Singly Linked List

• Nodes connected in a chain by links

• The head of the list is the front of the queue, the tail of the list is the rear of the queue. Why not the opposite?

Removing at the Head

Inserting at the Tail

Double-Ended Queues

• A double-ended queue, or deque, supports insertion and deletion from the front and back.
• The Deque Abstract Data Type
• insertFirst(e): Insert e at the deginning of deque.
• insertLast(e): Insert e at end of deque
• removeFirst(): Removes and returns first element
• removeLast(): Removes and returns last element
• Additionally supported methods include:
• first()
• last()
• size()
• isEmpty()

Implementing Stacks and Queues with Deques

• Stacks with Deques:

• Queues with Deques:

The Adaptor Pattern

• Using a deque to implement a stack or queue is an example of the adaptor pattern. Adaptor patterns implement a class by using methods of another class
• In general, adaptor classes specialize general classes
• Two such applications:
• Specialize a general class by changing some methods.
• Ex: implementing a stack with a deque.
• Specialize the types of objects used by a general class.
• Ex: Defining an IntegerArrayStack class that adapts ArrayStack to only store integers.

Implementing Deques with Doubly Linked Lists

• Deletions at the tail of a singly linked list cannot be done in constant time.
• To implement a deque, we use a doubly linked list. with special header and trailer nodes

• A node of a doubly linked list has a next and a prev link. It supports the following methods:
• setElement(Object e)
• setNext(Object newNext)
• setPrev(Object newPrev)
• getElement()
• getNext()
• getPrev()
• By using a doubly linked list, all the methods of a deque run in O(1) time.

Implementing Deques with Doubly Linked Lists (cont.)

• When implementing a doubly linked lists, we add two special nodes to the ends of the lists: the header and trailer nodes.
• The header node goes before the first list element. It has a valid next link but a null prev link.
• The trailer node goes after the last element. It has a valid prev reference but a null next reference.

• NOTE: the header and trailer nodes are sentinel or “dummy” nodes because they do not store elements. Here’s a diagram of our doubly
• linked list:

Implementing Deques with Doubly Linked Lists (cont.)

• Here’s a visualization of the code for removeLast().