Method shiftUp

interview/heap.js:16–22

(k)

    this.shiftUp(this.arr.length-1)
  }
  shiftUp(k){
    let {arr,compare,parent} = this
    while(k>1 && compare(arr[k],arr[parent(k)])){
      this.swap(parent(k),k)
      k = parent(k)
    }
  }
  pop(){
    // 弹出堆顶
    if(this.arr.length==1) return null