# 24. create a Priority Queue in JavaScript

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medium - accepted / - tried

Priority Queue is a commonly used data structure in algorithm problem. Especially useful for **Top K** problem with a huge amount of input data, since it could avoid sorting the whole but keep a fixed-length sorted portion of it.

Since there is no built-in Priority Queue in JavaScript, in a real interview, you should tell interview saying that "Suppose we already have a Priority Queue Class I can use", there is no time for you to write a Priority Queue from scratch.

But it is a good coding problem to practice, so please implement a Priority Queue with following interface

```
class PriorityQueue {
// compare is a function defines the priority, which is the order
// the elements closer to first element is sooner to be removed.
constructor(compare) {
}
// add a new element to the queue
// you need to put it in the right order
add(element) {
}
// remove the head element and return
poll() {
}
// get the head element
peek() {
}
// get the amount of items in the queue
size() {
}
}
```

Here is an example to make it clearer

```
const pq = new PriorityQueue((a, b) => a - b)
// (a, b) => a - b means
// smaller numbers are closer to index:0
// which means smaller number are to be removed sooner
pq.add(5)
// now 5 is the only element
pq.add(2)
// 2 added
pq.add(1)
// 1 added
pq.peek()
// since smaller number are sooner to be removed
// so this gives us 1
pq.poll()
// 1
// 1 is removed, 2 and 5 are left
pq.peek()
// 2 is the smallest now, this returns 2
pq.poll()
// 2
// 2 is removed, only 5 is left
```