freeCodeCamp/guide/english/certifications/coding-interview-prep/data-structures/create-a-queue-class/index.md

---
title: Create a Queue Class
---
# Create a Queue Class


---
## Problem Explanation
- A Queue is an abstract Data Structure.
- A Queue folow FIFO/LILO principle.
- In this challenge we nede to implement `enqueue()`, `dequeue()`, `front()`, `size()`, `isEmpty()` methods.
  - `enqueue()` - This method adds the element to the queue.
  - `dequeue()` - This method removes the first element from the queue.
  - `front()` - This method returns the first element in the queue that'd be dequeue'd.
  - `size()` - This method returns the size of the queue.
  - `isEmpty()` - This method returns if the queue is empty.
- 
  | DS    | Access | Search | Insert | Delete |
  | ----- | ------ | ------ | ------ | ------ |
  | Queue |   n    |    n   |   1    |    1   |
  
 - ![Queue in action](https://upload.wikimedia.org/wikipedia/commons/thumb/5/52/Data_Queue.svg/405px-Data_Queue.svg.png)


---
## Solutions

<details><summary>Solution 1 (Click to Show/Hide)</summary>

**Note:** This solution is not exactly a queue, the shift() method used in the dequeue() method is of complexity `O(n)` and not `O(1)`. However, the advanced solution rectifies this and uses Object(HashTables) instead of Array to implement Queue. 

```js
function Queue() {
  var collection = [];
  this.print = function() {
    console.log(collection);
  };
  this.enqueue = function(val) {
    collection.push(val);
  };
  this.dequeue = function() {
    return collection.shift();
  };
  this.front = function() {
    return collection[0];
  };
  this.size = function() {
    return collection.length;
  };
  this.isEmpty = function() {
    return collection.length === 0;
  };
}
```
</details>


<details><summary>Solution 2 (Click to Show/Hide)</summary>

```js
class Queue {
  constructor() {
    this.collection = {};
    this.start = 0;
    this.end = 0;
  }
  print() {
    console.log(this.collection);
  }
  enqueue(val) {
    this.collection[this.end++] = val;
  }
  dequeue() {
    return this.collection[this.start++];
  }
  front() {
    return this.collection[this.start];
  }
  size() {
    return this.end - this.start;
  }
  isEmpty() {
    return this.size() === 0;
  }
}
```
#### Relevant Links
- [Wikipedia](https://en.wikipedia.org/wiki/Queue_(abstract_data_type))
</details>