Intro
After we finished the small series about the Stack, we start with the Queue.
What is a Queue?
- uses the "First In, First Out"-Principle
- Examples: a queue of people in front of a store, a printer queue
- there are multiple ways to implement a Queue: Array, Singly Linked List, Doubly Linked List
Big O of Queue
- Access:
O(N)
- Search:
O(N)
- Insert:
O(1)
- Delete:
O(1)
Example
We will use a Singly Linked List to build our Queue.
A (start) ==> B (end)
- we can enqueue (= add) to the end (e.g. a new person will be the last person in the queue)
- we can dequeue (= remove) from the start (e.g. a person at the start gets served next)
-
A
is the next node in line -
A
has a pointer (next
) to the next node (B
) -
B
is the last node we enqueued (= added) to the Queue - if we dequeue (= remove)
A
, the next node in line should beB
Setup
We need the following parts to build our Queue:
- a Node with a value and a pointer to the next item in the Queue
- a Queue with a length, a pointer to the start of the queue, a pointer to the end of the queue
// a Node has a value (`value`) and a pointer to the next node (`next`)
class Node {
constructor(value) {
this.value = value;
this.next = null;
}
}
// a Queue has a length (`length`), a start (`start`), an end (`end`)
class Queue {
constructor() {
this.length = 0;
this.start = null;
this.end = null;
}
}
Thoughts
We set up our Queue. Now we need at least two methods within the Queue:
- a method that adds a new node to the end of the Queue:
enqueue
- a method that removes a node from the start of the Queue:
dequeue
Next Part
We will implement our first method for the Queue.
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Questions
- We could also use an Array to build a Queue. How could we do this? Any pros or cons?
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