freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/data-structures/create-a-circular-queue.english.md

id, title, challengeType

id	title	challengeType
587d8255367417b2b2512c75	Create a Circular Queue	1

Description

In this challenge you will be creating a Circular Queue. A circular queue is a queue that writes to the end of a collection then begins overwriting itself at the beginning of the collection. This type of data structure is useful in certain situations. For example, a circular queue can be used for streaming media. Once the queue is full, new media data will overwrite old data.

A good way to illustrate this concept is with an array of length 5:

[null, null, null, null, null]
 ^Read @ 0
 ^Write @ 0

Here the read and write are both at position 0. Now the queue gets 3 new records a, b, and c. Our queue now looks like:

[a, b, c, null, null]
 ^Read @ 0
          ^Write @ 3

As the read head reads, it can remove values or keep them:

[null, null, null, null, null]
                   ^Read @ 3
                   ^Write @ 3

Now we write the values d, e, and f to the queue. Once the write reaches the end of the array it loops back to the beginning:

[f, null, null, d, e]
                ^Read @ 3
    ^Write @ 1

This approach requires a constant amount of memory but allows files of a much larger size to be processed.

Instructions

In this challenge we will implement a circular queue. The circular queue should provide `enqueue` and `dequeue` methods which allow you to read from and write to the queue. The class itself should also accept an integer argument which you can use to specify the size of the queue when created. We've written the starting version of this class for you in the code editor. When you enqueue items to the queue, the write pointer should advance forward and loop back to the beginning once it reaches the end of the queue. Likewise, the read pointer should advance forward as you dequeue items. The write pointer should not be allowed to move past the read pointer (our class won't let you overwrite data you haven't read yet) and the read pointer should not be able to advance past data you have written. In addition, the `enqueue` method should return the item you enqueued if it is successful; otherwise it will return `null`. Similarly, when you dequeue an item, that item should be returned and if you cannot dequeue an item you should return `null`.

Tests

tests:
  - text: The <code>enqueue</code> method adds items to the circular queue.
    testString: assert((function(){ var test = new CircularQueue(3); test.enqueue(17); test.enqueue(32); test.enqueue(591); var print = test.print(); return print[0] === 17 && print[1] === 32 && print[2] === 591; })(), 'The <code>enqueue</code> method adds items to the circular queue.');
  - text: You cannot enqueue items past the read pointer.
    testString: assert((function(){ var test = new CircularQueue(3); test.enqueue(17); test.enqueue(32); test.enqueue(591); test.enqueue(13); test.enqueue(25); test.enqueue(59); var print = test.print(); return print[0] === 17 && print[1] === 32 && print[2] === 591; })(), 'You cannot enqueue items past the read pointer.');
  - text: The <code>dequeue</code> method dequeues items from the queue.
    testString: assert((function(){ var test = new CircularQueue(3); test.enqueue(17); test.enqueue(32); test.enqueue(591); return test.dequeue() === 17 && test.dequeue() === 32 && test.dequeue() === 591; })(), 'The <code>dequeue</code> method dequeues items from the queue.');
  - text: After an item is dequeued its position in the queue should be reset to <code>null</code>.
    testString: assert((function(){ var test = new CircularQueue(3); test.enqueue(17); test.enqueue(32); test.enqueue(672); test.dequeue(); test.dequeue(); var print = test.print(); return print[0] === null && print[1] === null && print[2] === 672; })(), 'After an item is dequeued its position in the queue should be reset to <code>null</code>.');
  - text: Trying to dequeue past the write pointer returns <code>null</code> and does not advance the write pointer.
    testString: assert((function(){ var test = new CircularQueue(3); test.enqueue(17); test.enqueue(32); test.enqueue(591); return test.dequeue() === 17 && test.dequeue() === 32 && test.dequeue() === 591 && test.dequeue() === null && test.dequeue() === null && test.dequeue() === null && test.dequeue() === null && test.enqueue(100) === 100 && test.dequeue() === 100; })(), 'Trying to dequeue past the write pointer returns <code>null</code> and does not advance the write pointer.');

Challenge Seed

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) {
    // Only change code below this line

    // Only change code above this line
   }

   dequeue() {
    // Only change code below this line

    // Only change code above this line
   }
}

Solution

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) {
    // Only change code below this line
    console.log(this.write, this.max);
    if (this.queue[this.write] === null) {
      this.queue[this.write++] = item;

      if (this.write > this.max) {
        this.write = 0;
      }
      return item;
    }
    return null;
    // Only change code above this line
  }

  dequeue() {
    // Only change code below this line
    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;
    // Only change code above this line
  }
}