题目：

题解：

bool check(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(int **matrix, int matrixSize, int *matrixColSize, int k) {
    int left = matrix[0][0];
    int right = matrix[matrixSize - 1][matrixSize - 1];
    while (left < right) {
        int mid = left + ((right - left) >> 1);
        if (check(matrix, mid, k, matrixSize)) {
            right = mid;
        } else {
            left = mid + 1;
        }
    }
    return left;
}