_siftUp() {
    let i;
    let parent;

    for (
      i = this.n;
      i > 1 &&
      (parent = i >> 1) &&
      this._comparator.greaterThan(this._elements[parent], this._elements[i]);
      i = parent
    ) {
      this._swap(parent, i);
    }
  }
Sift up the last element
O(lg n)