動機

reverse iterator好讚

Problem

Given a m x n matrix grid which is sorted in non-increasing order both row-wise and column-wise, return the number of negative numbers in grid.

 

Example 1:

Input: grid = [[4,3,2,-1],[3,2,1,-1],[1,1,-1,-2],[-1,-1,-2,-3]]Output: 8Explanation: There are 8 negatives number in the matrix.

Example 2:

Input: grid = [[3,2],[1,0]]Output: 0

Example 3:

Input: grid = [[1,-1],[-1,-1]]Output: 3

Example 4:

Input: grid = [[-1]]Output: 1

 

Constraints:

    t
  • m == grid.length
    • t
  • n == grid[i].length
    • t
  • 1 <= m, n <= 100
    • t
  • -100 <= grid[i][j] <= 100

 

Follow up: Could you find an O(n + m) solution?

Sol

class Solution {
public:
    int countNegatives(vector<vector<int>>& grid) {
        int ret = 0;
        for (auto& r:grid)
            ret += (lower_bound(r.rbegin(),r.rend(),0)-r.rbegin());
        return ret;
    }
};