LeetCode题解：Set Matrix Zeroes

Set Matrix Zeroes

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

思路：

如果允许额外存储行和列清零的信息，那么题目就没有意思了。希望只用O(1)的存储空间的时候，我们就得利用矩阵本身的存储空间了：把清零的信息保存在第一行和第一列里。

题解：

class Solution {
public:
    void setZeroes(vector<vector<int> > &matrix) {
        const int ROW = matrix.size();
        if (ROW == 0)
            return;
        const int COL = matrix[0].size();
        if (COL == 0)
            return;
        
        // we store the 0 information in
        // the 1st row and 1st col
        bool R00 = false;
        for(int i = 0; i < COL; ++i)
            if (matrix[0][i] == 0)
            {
                R00 = true;
                break;
            }
        bool C00 = false;
        for(int i = 0; i < ROW; ++i)
            if (matrix[i][0] == 0)
            {
                C00 = true;
                break;
            }
        
        // now traverse the remaining parts of
        // the matrix and store 0 into the 
        // first row
        for(int i = 1; i < ROW; ++i)
            for(int j = 1; j < COL; ++j)
                if (matrix[i][j] == 0)
                {
                    matrix[i][0] = 0;
                    matrix[0][j] = 0;
                }
                
        // use the first row/col information to
        // fill zeros
        for(int i = 1; i < ROW; ++i)
            if (matrix[i][0] == 0)
                for(int j = 1; j < COL; ++j)
                    matrix[i][j] = 0;
        
        for(int i = 1; i < COL; ++i)
            if (matrix[0][i] == 0)
                for(int j = 1; j < ROW; ++j)
                    matrix[j][i] = 0;
        
        // Finally check the 1st row/col
        if (R00)
            fill(begin(matrix[0]), end(matrix[0]), 0);
        if (C00)
            for(int i = 0; i < ROW; ++i)
                matrix[i][0] = 0;
        
        return;
    }
};