面试题 17.14. 最小K个数

用时 ：10min

const swap = function (arr,i,j) {
    [arr[i],arr[j]] = [arr[j],arr[i]]
}

class MaxHeap {
    constructor() {
        this.count = 0
        this.data = new Array(this.count + 1)
    }
    shiftUp(k) {
        // 把新的元素往上排
        while(k>=1) {
            let father = Math.floor(k / 2)
            if (this.data[k] > this.data[father]) {
                swap(this.data,k,father)
                k = father
            } else {
                break
            }
        }
    }
    shiftDown(k) {
        while( k * 2  <= this.count) { // 表示k 有孩子
            let j = k
            if (k * 2 + 1 <= this.count && this.data[k * 2 + 1] > this.data[k] && this.data[k * 2 + 1] > this.data[k * 2]) {
                j = k * 2 + 1
            } else if (this.data[k * 2] > this.data[k]) {
                j = k * 2
            } else {
                break
            }
            swap(this.data,j,k)
            k = j
        }
    }
    size() {
        return this.count
    }
    isEmpty() {
        return this.count === 0
    }
    insert(item) {
        this.data[++this.count] = item 
        this.shiftUp(this.count)
    }
    extractMax() {
        if (this.count < 0) return 
        let ret = this.data[1]
        swap(this.data,1,this.count--)
        this.shiftDown(1)
        return ret
    }
    top() {
        return this.data[1]
    }
}


var smallestK = function(arr, k) {
    var heap = new MaxHeap()
    var K = 0
    
    while(K < k) {
        heap.insert(arr[K])
        K++
    }
    // 将堆维持在K的大小
    while(K < arr.length) {
        var max = heap.top()
        if (arr[K] < max) {
            heap.extractMax()
            heap.insert(arr[K])
        }
        K++
    }
    var size = heap.size()
    return heap.data.slice(1, 1 + size )
};