1351. Count Negative Numbers in a Sorted Matrix 

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class Solution {
    public int countNegatives(int[][] grid) {
        //  4  3  2  -1
        //  3  2  1  -1
        //  1  1 -1  -2
        // -1 -1 -2  -3

        int m = grid.length, n = grid[0].length;
        
        int count = 0;
        int i = 0, j = n - 1;
        while (i < m) {
            // 从右向左寻找该行最右侧的非负数数字的索引
            while (j >= 0 && grid[i][j] < 0) {
                j--;
            }

            count += n - j - 1;
            i++;
        }

        return count;
    }
}

References

1351. Count Negative Numbers in a Sorted Matrix