[Guide] Data Structures: Create a Priority Queue Class (#19618)

* feat: added priority queue

* Corrected a few misspellings, added some white space between functions

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

@@ -3,8 +3,123 @@ title: Create a Priority Queue Class
 ---
 ## Create a Priority Queue Class
 
-This is a stub. <a href='https://github.com/freecodecamp/guides/tree/master/src/pages/certifications/coding-interview-prep/data-structures/create-a-priority-queue-class/index.md' target='_blank' rel='nofollow'>Help our community expand it</a>.
+### Method:
+- Priority Queue is an Abstract Data Type.
+- It can be implemented using other Data Structures but is commonly implemented using a Heap.
+- Each node contains a priority. When we enqueue a node to the queue, it's "bubbled up" to its place in the queue.
+- In this challenge we need to create a Priority Queue class based on Min-Heap with a few methods:
+  - enqueue() - It enqueue's the new node to it's appropriate place in queue.
+  - dequeue() - It removes the first node from the queue.
+  - front() - This method returns the first node to be dequeued.
+  - size() - Returns the size of the queue.
+  - isEmpty() - Returns if the queue is empty.
+- | DS    | Access | Search | Insert | Delete |
+  | ----- | ------ | ------ | ------ | ------ |
+  | Priority Queue |   1    |    1   |   logn    |    logn   |
 
-<a href='https://github.com/freecodecamp/guides/blob/master/README.md' target='_blank' rel='nofollow'>This quick style guide will help ensure your pull request gets accepted</a>.
 
-<!-- The article goes here, in GitHub-flavored Markdown. Feel free to add YouTube videos, images, and CodePen/JSBin embeds  -->
+### Solution:
+```js
+//function which swaps elements in array, using ES6 syntax
+function swap(arr, i, j) {
+  [arr[i], arr[j]] = [arr[j], arr[i]];
+}
+
+class PriorityQueue {
+  constructor() {
+    //initializing queue
+    this.collection = [];
+  }
+
+  printCollection() {
+    console.log(this.collection);
+  }
+
+  //enqueues the node to the queue
+  enqueue(node) {
+    this.collection.push(node);
+    //calling the helper method which bubbles up the node to the correct position
+    this.bubbleUp();
+  }
+
+  //helper method which bubbles the node to its correct position
+  bubbleUp() {
+    let i = this.size() - 1;
+    let node = this.collection[i];
+
+    while (true) {
+      //getting the parent node's index
+      let parent = Math.floor((i - 1) / 2);
+      if (parent < 0) return;
+
+      //comparing with parent, if the node is smaller then swapping
+      if (this.collection[parent][1] > node[1]) {
+        swap(this.collection, parent, i);
+        i = parent;
+      } else break;
+    }
+  }
+
+  //dequeues the node and returns its value
+  dequeue() {
+    if (this.isEmpty()) return;
+    if (this.size() === 1) {
+      return this.collection.pop()[0];
+    }
+    //getting the first element from the queue
+    let pop = this.collection[0];
+
+    //replacing the first element by last
+    this.collection[0] = this.collection.pop();
+
+    //calling the buubleDown method which bubbles down the node to the correct position
+    this.bubbleDown();
+    return pop[0];
+  }
+
+  //helper method which bubble's down the node to the correct position
+  bubbleDown() {
+    let node = this.collection[0];
+    let i = 0;
+
+    while (true) {
+      //getting the two child node's index
+      let child1 = 2 * i + 1;
+      let child2 = 2 * i + 2;
+
+      //edge case
+      if (child2 >= this.size()) return;
+
+      //getting the minimum of two child
+      let min =
+        this.collection[child1][1] > this.collection[child2][1]
+          ? child2
+          : child1;
+
+      //if the child is smaller than node then swap
+      if (this.collection[min][1] < node[1]) {
+        swap(this.collection, i, min);
+        i = min;
+      } else break;
+    }
+  }
+
+  //returns the first element from queue
+  front() {
+    return this.collection[0];
+  }
+
+  // returns the size of the queue
+  size() {
+    return this.collection.length;
+  }
+
+  // returns if the queue is empty
+  isEmpty() {
+    return this.size() === 0;
+  }
+}
+```
+- [Run Code](https://repl.it/@ezioda004/Priority-Queue)
+### References:
+- [Wikipedia](https://en.wikipedia.org/wiki/Priority_queue)