[LeetCode] Set Matrix Zeroes

This problem can be solved easily if we are allowed to use more than O(1) space. For example, you may create a copy of the original matrix (O(mn)-space) or just record the row and column indexes (O(m + n)-space). Well, is there a O(1)-space solution? Yes, you may refer to this link :-)

The key idea is to record the rows and columns that need to be set to zeroes directly in the matrix ( if (!matrix[i][j]) matrix[i][0] = matrix[0][j] = 0; ). The code is rewritten as follows.

class Solution {
public:
    void setZeroes(vector<vector<int>>& matrix) {
        int m = matrix.size(), n = matrix[0].size();
        bool c0 = false;
        for (int i = 0; i < m; i++) {
            if (!matrix[i][0]) c0 = true;
            for (int j = 1; j < n; j++)
                if (!matrix[i][j]) matrix[i][0] = matrix[0][j] = 0;
        }
        for (int i = m - 1; i >= 0; i--) {
            for (int j = n - 1; j; j--)
                if (!matrix[i][0] || !matrix[0][j]) matrix[i][j] = 0;
            if (c0) matrix[i][0] = 0;
        }
    }
};