class Solution {
public int kthSmallest(int[][] matrix, int k) {
int n = matrix.length;
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;
}
public boolean 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;
}
}
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