Definition

Queues are a data type or collection in which the collection maintains a particular oder in which the elements are added and removed. Queues follow First-In-First-Out (FIFO) principle. The first element added to the queue is the first one to be removed.

We should be able to...	Run This
Create a new, empty queue	`const queue = new Queue();`
Add an element to the end of the queue	`queue.add(1);`
Remove the element from the front of the queue	`queue.remove();`
Check if a queue is empty	`isEmpty()`

Runtime

Method	Time Complexity
`lookup`	O(n)
`enqueue`	O(1)
`dequeue`	O(1)
`isEmpty`	O(1)
`peek`	O(1)

big o

Queue With Arrays

class Queue {
  constructor() {
    this.data = []
  }

  enqueue(item) {
    this.data.push(item)
  }

  dequeue(item) {
    return this.data.shift()
  }

  peek(item) {
    return this.data[0]
  }

  isEmpty() {
    return this.data.length === 0
  }
}

// Test
const queue = new Queue()

queue.isEmpty() // true

queue.enqueue('A')
queue.enqueue('B')
queue.enqueue('C')
queue.enqueue('D')
queue.enqueue('E')

queue.isEmpty() // false

queue.peek() // 'A'

queue.dequeue() // 'A'
queue.dequeue() // 'B'
queue.dequeue() // 'C'

Create a class with a constructor that initializes an empty array of data.
Define an enqueue() method to add an element to the end of the data array.
Define a dequeue() method to remove an element from the start of the data array.
Define a peek() method, which retrieves the value of the first element in the data array, without removing it.
Define an isEmpty() method to determine if the data array is empty.

Queues without Arrays

class Node {
  constructor(value) {
    this.value = value
    this.next = null
  }
}

// first in, first out
class Queue {
  constructor() {
    this.first = null
    this.last = null
    this.length = 0
  }

  peek() {
    return this.first
  }

  enqueue(value) {
    const newItem = new Node(value)

    if (this.length === 0) {
      this.first = newItem
      this.last = newItem
    } else {
      this.last.next = newItem
      this.last = newItem
    }
    this.length++
    return this
  }

  dequeue() {
    if (!this.first) {
      return null
    }

    if (this.first === this.last) {
      this.last = null
    }

    const temp = this.first
    this.first = this.first.next
    this.length--
    return temp
  }

  isEmpty() {
    return this.length ? false : true
  }
}