题目：

题解：

class Solution {
public:bool check(vector<vector<int>>& matrix, int mid, int k, int n) {int i = n - 1;int j = 0;int num = 0;while (i >= 0 && j < n) {if (matrix[i][j] <= mid) {num += i + 1;j++;} else {i--;}}return num >= k;}int kthSmallest(vector<vector<int>>& matrix, int k) {int n = matrix.size();int left = matrix[0][0];int right = matrix[n - 1][n - 1];while (left < right) {int mid = left + ((right - left) >> 1);if (check(matrix, mid, k, n)) {right = mid;} else {left = mid + 1;}}return left;}
};