73. Set Matrix Zeroes

不难想到在遍历时记录下哪些行和列需要被置0。为了节约空间，我们可以把第0行和列用作记录，再额外用两个bool记录它们俩需不需要被置0。

/*
 * @lc app=leetcode id=73 lang=cpp
 *
 * [73] Set Matrix Zeroes
 */
class Solution {
public:
    void setZeroes(vector<vector<int>>& matrix) {
        const int M = matrix.size(), N = M == 0 ? 0 : matrix[0].size();
        bool r0 = false, c0 = false;
        for (int i = 0; i < M; ++i) {
            for (int j = 0; j < N; ++j) {
                if (matrix[i][j] == 0) {
                    matrix[i][0] = matrix[0][j] = 0;
                    if (i == 0) r0 = true;
                    if (j == 0) c0 = true;
                }
            }
        } 
        for (int i = 1; i < M; ++i) {
            for (int j = 1; j < N; ++j) {
                if (matrix[i][0] == 0 || matrix[0][j] == 0) {
                    matrix[i][j] = 0;
                }
            }
        }
        if (r0) {
            for (int j = 0; j < N; ++j) matrix[0][j] = 0;
        }
        if (c0) {
            for (int i = 0; i < M; ++i) matrix[i][0] = 0;
        }
    }
};