在2022年二月份有很多前端的朋友面试大厂会问到了js 判断数组第二大的数的题（其实就是一道算法题，在力扣题号为：215，题目为：数组中的第K个最大元素。

<script type="text/javascript">
//访问网址：https://xuexiluxian.cn/
var findKthLargest = function(nums, k) {
    let arr = new MinHeap();
    nums.forEach(item=>{
        arr.insert( item );
        if( arr.size() > k ){
            arr.pop();
        }
    })
    return arr.peek();
};
findKthLargest([3,2,1,5,6,4],2);

class MinHeap{
    constructor(){
        this.heap = [];
    }
    //换位置
    swap(i1,i2){
        const temp = this.heap[i1];
        this.heap[i1] = this.heap[i2];
        this.heap[i2] = temp;
    }
    //找到父节点
    getParentIndex( index ){
        return Math.floor( (index-1)/2  );
    }
    //上(前)移操作
    up( index ){
        //如果是0就不移动了
        if( index ==0 )return;
        const parentIndex = this.getParentIndex( index );
        //如果父元素大于当前元素,就开始移动
        if( this.heap[parentIndex] > this.heap[index] ){
            this.swap(  parentIndex  ,  index );
            this.up( parentIndex );
        }
    } 
    //获取左侧子节点
    getLeftIndex( index ){
        return index * 2 + 1;
    }
    //获取右侧子节点
    getRightIndex( index ){
        return index * 2 + 2;
    }
    //下(后)移操作
    down( index ){
        const leftIndex = this.getLeftIndex( index );
        const rightIndex = this.getRightIndex( index );
        if( this.heap[leftIndex] < this.heap[index] ){
            this.swap(leftIndex,index);
            this.down( leftIndex );
        }
        if( this.heap[rightIndex] < this.heap[index] ){
            this.swap(rightIndex,index);
            this.down( rightIndex );
        }
    }
    //添加元素
    insert( value ){
        this.heap.push( value );
        this.up( this.heap.length-1 );
    }
    //删除堆顶
    pop(){
        this.heap[0] = this.heap.pop();
        this.down( 0 );
    }
    //获取堆顶
    peek(){
        return this.heap[0]
    }
    size(){
        return this.heap.length;
    }

}
</script>