LeetCode Set Matrix Zeroes

Did you use extra space?
A straight forward solution using O(mn) space is probably a bad idea.
A simple improvement uses O(m + n) space, but still not the best solution.
Could you devise a constant space solution?

题意：将0的行列置0.

思路：为了不开额外的空间，我们借助第一行和第一列，然后判断是否第一行、列也要置0

class Solution {
public: 
	void setZeroes(vector<vector<int> > &matrix) {
		int n = matrix.size();
		int m = matrix[0].size();

		bool firstRow = false, firstCol = false;
		for (int i = 0; i < n; i++) 
			if (matrix[i][0] == 0) {
				firstCol = true;
				break;
			}
		for (int i = 0; i < m; i++) 
			if (matrix[0][i] == 0) {
				firstRow = true;
				break;
			}

		for (int i = 1; i < n; i++) {
			for (int j = 1; j < m; j++) {
				if (matrix[i][j] == 0) {
					matrix[i][0] = 0;
					matrix[0][j] = 0;
				}
			}
		}

		for (int i = 1; i < n; i++) {
			if (matrix[i][0] == 0)
				for (int j = 1; j < m; j++)
					matrix[i][j] = 0;
		}
		for (int i = 1; i < m; i++) {
			if (matrix[0][i] == 0)
				for (int j = 1; j < n; j++)
					matrix[j][i] = 0;
		}

		if (firstRow) {
			for (int i = 0; i < m; i++)
				matrix[0][i] = 0;
		}
		if (firstCol) {
			for (int i = 0; i < n; i++)
				matrix[i][0] = 0;
		}
	}
};