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

---
title: Create a Circular Queue
---
# Create a Circular Queue

---
## Problem Explanation
- In this challenge we create a Circular Queue data structure.
- First, we need to create an array of `size` with all elements set to `null`.
- Then we create an equeue method, which moves the write pointer but doesnt exceed the read pointer.
- The dequeue method on the other hand, moves the read pointer but doesnt exceed the write pointer.
- Example:

  - First, we create an array of length 5:
```
  [null, null, null, null, null]
   ^Read @ 0
   ^Write @ 0
```
 - Then we enqueue `a`, `b`, and `c`:
```
  [a, b, c, null, null]
   ^Read @ 0
            ^Write @ 3
```
  - Now we dequeue all the enqueued items:

```
  [null, null, null, null, null]
                     ^Read @ 3
                     ^Write @ 3
```
  - Finally, we enqueue `d`, `e` and `f`:
```
  [f, null, null, d, e]
                  ^Read @ 3
      ^Write @ 1
```


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

```js
class CircularQueue {
  constructor(size) {
    this.queue = [];
    this.read = 0;
    this.write = 0;
    this.max = size - 1;

    while (size > 0) {
      this.queue.push(null);
      size--;
    }
  }

  print() {
    return this.queue;
  }

  enqueue(item) {
    if (this.queue[this.write] === null) {
      this.queue[this.write++] = item;

      if (this.write > this.max) this.write = 0;
      return item;
    }
    return null;
  }

  dequeue() {
    if (this.queue[this.read] != null) {
      let item = this.queue[this.read];
      this.queue[this.read++] = null;
      if (this.read > this.max) this.read = 0;
      return item;
    }
    return null;
  }
}
```
#### Relevant Links

- [Wikipedia](https://en.wikipedia.org/wiki/Circular_buffer)

</details>