思想，利用快排思想，不断寻找分解点，将分界点的下标与K-1比较如果相等，返回该值，否则更新左右边界。当左右边界中的值少于等于2个时，运用插入排序，返回a[k-1]

int findCut(vector
     
      & a, int l, int r){	if (l + 1 >= r)	{		return -1;	}	int pivot = a[l];	int i = l, j = r;	while (true)	{		do		{			i++;		} while (i
      
       < pivot);		do		{			j--;		} while (j>l&&a[j] >= pivot);		if (i >= j)break;		int temp = a[i];		a[i] = a[j];		a[j] = temp;	}	a[l] = a[j];	a[j] = pivot;	return j;}void insert_sort(vector
       
        &a, int l, int r){	for (int i = l + 1; i < r; i++)	{		int j=i,pivot = a[i];		while (j>l&&pivot < a[j-1])		{			a[j] = a[j - 1];			j--;		}		a[j] = pivot;	}}int findKth(vector
        
         a, int n, int K){	int left = 0, right = n;	while (left+1 < right)	{		int cut = findCut(a, left, right);		if (cut == K - 1)return a[cut];		else if (cut < K - 1)		{			left = cut + 1;		}		else		{			right = cut;		}	}	insert_sort(a, left, right);	return a[K-1];}