剑指 Offer II 059. 数据流的第 K 大数值

剑指 Offer II 059. 数据流的第 K 大数值

我们可以使用一个大小为 k 的优先队列来存储前 k 大的元素，其中优先队列的队头为队列中最小的元素，也就是第 k 大的元素。

在单次插入的操作中，我们首先将元素 val 加入到优先队列中。如果此时优先队列的大小大于 k，我们需要将优先队列的队头元素弹出，以保证优先队列的大小为 k。

var KthLargest = function (k, nums) {
  this.k = k;
  this.heap = new MinHeap();
  for (const x of nums) {
    this.add(x);
  }
};
KthLargest.prototype.add = function (val) {
  this.heap.offer(val);
  if (this.heap.size() > this.k) {
    this.heap.poll();
  }
  return this.heap.peek();
};

class MinHeap {
  constructor(data = []) {
    this.data = data;
    this.comparator = (a, b) => a - b;
    this.heapify();
  }
  heapify() {
    if (this.size() < 2) return;
    for (let i = 1; i < this.size(); i++) {
      this.bubbleUp(i);
    }
  }
  peek() {
    if (this.size() === 0) return null;
    return this.data[0];
  }
  offer(value) {
    this.data.push(value);
    this.bubbleUp(this.size() - 1);
  }
  poll() {
    if (this.size() === 0) {
      return null;
    }
    const result = this.data[0];
    const last = this.data.pop();
    if (this.size() !== 0) {
      this.data[0] = last;
      this.bubbleDown(0);
    }
    return result;
  }
  bubbleUp(index) {
    while (index > 0) {
      const parentIndex = (index - 1) >> 1;
      if (this.comparator(this.data[index], this.data[parentIndex]) < 0) {
        this.swap(index, parentIndex);
        index = parentIndex;
      } else {
        break;
      }
    }
  }
  bubbleDown(index) {
    const lastIndex = this.size() - 1;
    while (true) {
      const leftIndex = index * 2 + 1;
      const rightIndex = index * 2 + 2;
      let findIndex = index;
      if (
        leftIndex <= lastIndex &&
        this.comparator(this.data[leftIndex], this.data[findIndex]) < 0
      ) {
        findIndex = leftIndex;
      }
      if (
        rightIndex <= lastIndex &&
        this.comparator(this.data[rightIndex], this.data[findIndex]) < 0
      ) {
        findIndex = rightIndex;
      }
      if (index !== findIndex) {
        this.swap(index, findIndex);
        index = findIndex;
      } else {
        break;
      }
    }
  }
  swap(index1, index2) {
    [this.data[index1], this.data[index2]] = [
      this.data[index2],
      this.data[index1],
    ];
  }
  size() {
    return this.data.length;
  }
}