constructor() {
    this.min = new Heap((a, b) => a - b);
    this.max = new Heap((a, b) => b - a);
  }
Median Heap using one MaxHeap and one MinHeap.

Each heap contains about one half of the data.
Every element in the min-heap is greater or equal to the median,
and every element in the max-heap is less or equal to the median.

@author Adrian Mejia <adrian@adrianmejia.com>