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17.PriorityQueue.js
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17.PriorityQueue.js
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// WHAT IS A PRIORITY QUEUE?
// A data structure where each element has a priority.
// Elements with higher priorities are served before elements with lower priorities.
// OUR PRIORITY QUEUE
// Write a Min Binary Heap - lower number means higher priority.
// Each Node has a val and a priority. Use the priority to build the heap.
// Enqueue method accepts a value and priority, makes a new node, and puts it in the right spot based off of its priority.
// Dequeue method removes root element, returns it, and rearranges heap using priority
class Node {
constructor(value, priority){
this.value = value;
this.priority = priority; // greater the priority number, higher is the priority
}
}
// MinBinaryHeap
class PriorityQueue {
constructor() {
this.values = [];
}
enqueue(value, priority) {
const node = new Node(value, priority);
this.values.push(node);
this.bubbleUp();
}
bubbleUp() {
let currentIdx = this.values.length - 1;
let currentValue = this.values[currentIdx];
while(currentIdx > 0) {
let parentIdx = Math.floor((currentIdx - 1) / 2)
let parentValue = this.values[parentIdx];
if(currentValue.priority > parentValue.priority) {
this.values[parentIdx] = currentValue;
this.values[currentIdx] = parentValue;
currentIdx = parentIdx;
} else {
break;
}
}
}
dequeue() {
const max = this.values[0];
const last = this.values.pop();
// edge case
if(this.values.length > 0) {
this.values[0] = last;
this.sinkDown();
}
return max;
}
sinkDown() {
let parentIdx = 0
let parentValue = this.values[parentIdx];
const length = this.values.length;
while(true) {
let leftChildIdx = (2 * parentIdx) + 1;
let rightChildIdx = (2 * parentIdx) + 2;
let leftChildValue, rightChildValue;
let swapIndex = null;
let swapValue = null;
if(leftChildIdx < length) {
leftChildValue = this.values[leftChildIdx]
if(leftChildValue.priority > parentValue.priority) {
swapIndex = leftChildIdx;
swapValue = leftChildValue
}
}
if(rightChildIdx < length) {
rightChildValue = this.values[rightChildIdx]
if((swapValue === null && rightChildValue.priority > parentValue.priority)
|| // leftChildValue.priority
(swapValue !== null && rightChildValue.priority > swapValue.priority)) {
swapIndex = rightChildIdx;
swapValue = rightChildValue
}
}
if(swapValue !== null) {
this.values[parentIdx] = swapValue;
this.values[swapIndex] = parentValue
parentIdx = swapIndex
} else {
break;
}
}
}
}
let ER = new PriorityQueue();
ER.enqueue("common cold", 5)
ER.enqueue("gunshot wound", 1)
ER.enqueue("high fever", 4)
ER.enqueue("broken arm", 2)
ER.enqueue("glass in foot", 3)
console.log(ER.dequeue()) // 5
console.log(ER.dequeue()) // 4
console.log(ER.dequeue()) // 3
console.log(ER.dequeue()) // 2
console.log(ER.dequeue()) // 1
console.log(ER.values)
// Simple or Naive Version.
// We are sorting which is O(n*logn)
// class PriorityQueue {
//
// constructor(){
// this.values = [];
// }
//
// enqueue(val, priority) {
// this.values.push({val, priority});
// this.sort();
// };
//
// dequeue() {
// return this.values.shift();
// };
//
// sort() {
// this.values.sort((a, b) => a.priority - b.priority);
// };
//
// }