堆与优先队列：TopK 与调度

// 运行环境：Node.js 14+
// 保存为 dsa-l13.js，执行：node dsa-l13.js
// MinHeap 实现见 dsa-l7.js，这里复用同款
class MinHeap {
  #h = []
  push(x) { this.#h.push(x); this.#siftUp(this.#h.length - 1) }
  pop() {
    if (!this.#h.length) return undefined
    const top = this.#h[0], last = this.#h.pop()
    if (this.#h.length) { this.#h[0] = last; this.#siftDown(0) }
    return top
  }
  peek() { return this.#h[0] }
  get size() { return this.#h.length }
  #siftUp(i) { while (i > 0) { const p = (i - 1) >> 1; if (this.#h[p] <= this.#h[i]) break; [this.#h[p], this.#h[i]] = [this.#h[i], this.#h[p]]; i = p } }
  #siftDown(i) { const n = this.#h.length; while (true) { let m = i, l = 2*i+1, r = 2*i+2; if (l < n && this.#h[l] < this.#h[m]) m = l; if (r < n && this.#h[r] < this.#h[m]) m = r; if (m === i) break; [this.#h[m], this.#h[i]] = [this.#h[i], this.#h[m]]; i = m } }
}

// TopK：维护大小为 k 的最小堆，遍历完堆顶即第 k 大
function topK(arr, k) {
  const heap = new MinHeap()
  for (const x of arr) {
    heap.push(x)
    if (heap.size > k) heap.pop() // 超过 k 就扔掉当前最小
  }
  return heap // 堆内即 TopK
}
const data = [3, 1, 5, 8, 2, 9, 4]
const h = topK(data, 3)
console.log([...Array(h.size)].map(() => h.pop())) // [9, 8, 5]（降序弹出）