QUEUES

OBJECTIVES

  • Define what a queue is
  • Understand use cases for a queue
  • Implement operations on a queue data structure

WHAT IS A QUEUE?

A FIFO data structure!

First In First Out

WE'VE SEEN THIS BEFORE

Queues exist everywhere! Think about the last time you waited in line....

  • Background tasks
  • Uploading resources
  • Printing / Task processing

How do we use them in programming?

BUILDING A QUEUE WITH AN ARRAY

A Queue Class

class Queue {
    constructor(){
        this.first = null;
        this.last = null;
        this.size = 0;
    }
}
class Node {
    constructor(value){
        this.value = value;
        this.next = null;
    }
}

How we'll visualize a queue

A series of nodes!

Let's see this in action!

10

2

22

7

last

first

size = 4

Enqueue

Adding to the beginning of the Queue!

Remember, queues are a FIFO data structure

Enqueue Pseudocode

  • This function accepts some value
  • Create a new node using that value passed to the function
  • If there are no nodes in the queue, set this node to be the first and last property of the queue
  • Otherwise, set the next property on the current last to be that node, and then set the last property of the queue to be that node
  • Increment the size of the queue by 1

YOUR

TURN

Dequeue

Removing from the beginning of the Queue!

Remember, queues are a FIFO data structure

Dequeue pseudocode

  • If there is no first property, just return null
  • Store the first property in a variable
  • See if the first is the same as the last (check if there is only 1 node). If so, set the first and last to be null
  • If there is more than 1 node, set the first property to be the next property of first 
  • Decrement the size by 1
  • Return the value of the node dequeued

YOUR

TURN

BIG O of QUEUES

Insertion -   O(1)

Removal -   O(1)

Searching -   O(N)

Access -   O(N)

RECAP

  • Queues are a FIFO data structure, all elements are first in first out.
  • Queues are useful for processing tasks and are foundational for more complex data structures
  • Insertion and Removal can be done in O(1)