Top K Frequent Elements

function topKFrequent(nums, k) {
  const count = {};
  for (let num of nums) {
    count[num] = (count[num] || 0) + 1;
  }

  const maxHeap = new MaxHeap(
    Object.keys(count),
    (a, b) => count[b] - count[a]
  );

  const result = [];

  for (let i = 0; i < k; i++) {
    result.push(maxHeap.extractMax());
  }

  return result;
}

class MaxHeap {
  constructor(data, comparator) {
    this.data = data;

    this.comparator = comparator;

    this.heapify();
  }

  heapify() {
    for (let i = Math.floor(this.data.length / 2) - 1; i >= 0; i--) {
      this.siftDown(i);
    }
  }

  siftDown(index) {
    let smallest = index;

    let left = 2 * index + 1;

    let right = 2 * index + 2;

    if (
      left < this.data.length &&
      this.comparator(this.data[smallest], this.data[left]) > 0
    ) {
      smallest = left;
    }

    if (
      right < this.data.length &&
      this.comparator(this.data[smallest], this.data[right]) > 0
    ) {
      smallest = right;
    }

    if (smallest !== index) {
      [this.data[index], this.data[smallest]] = [
        this.data[smallest],
        this.data[index],
      ];

      this.siftDown(smallest);
    }
  }

  extractMax() {
    if (this.data.length === 0) return null;

    if (this.data.length === 1) return this.data.pop();

    const max = this.data[0];

    this.data[0] = this.data.pop();

    this.siftDown(0);

    return max;
  }
}