leetcode-73. Set Matrix Zeroes

题目：

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place.
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?

这里的思路是先从左上到右下遍历，将所有需要需要置零的行和列的首位都置为0，然后再反向遍历将所有首位为0的行和列都置为0。这里还用一个值来表示首行是否需要置零。这也是为什么需要反向的原因。这个设计的非常漂亮。反正我是没想到。
答案来源

public class Solution {    public void setZeroes(int[][] matrix) {        int col0 = 1, rows = matrix.length, cols = matrix[0].length;        for (int i = 0; i < rows; i++) {            if (matrix[i][0] == 0) col0 = 0;            for (int j = 1; j < cols; j++)                if (matrix[i][j] == 0)                    matrix[i][0] = matrix[0][j] = 0;        }        for (int i = rows - 1; i >= 0; i--) {            for (int j = cols - 1; j >= 1; j--)                if (matrix[i][0] == 0 || matrix[0][j] == 0)                    matrix[i][j] = 0;            if (col0 == 0) matrix[i][0] = 0;        }    }}