LeetCode[Array]: Set Matrix Zeroes

标签：leetcode   array   matrix   算法   algorithm   

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

我的解题思路如下：遍历矩阵，分别用每一行的首元素保存该行的信息（若为0则表示需要将改行全部设为0），每一列的首元素保存该列的信息，之后根据这些信息进行设置。由于首行首列公用元素matrix[0][0]，所以另外设置两个标志row0和col0，分别表示是否需要将首行、首列置0。

我的C++代码实现如下：

void setZeroes(vector<vector<int> > &matrix) {
    int row0 = 1;
    for (int j = 0; j < matrix[0].size(); ++j)
        if (matrix[0][j] == 0) {
            row0 = 0;
            break;
        }

    int col0 = 1;
    for (int i = 0; i < matrix.size(); ++i)
        if (matrix[i][0] == 0) {
            col0 = 0;
            break;
        }

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

    for (int i = 1; i < matrix.size(); ++i)
        if (matrix[i][0] == 0)
            for (int j = 1; j < matrix[i].size(); ++j)
                matrix[i][j] = 0;

    for (int j = 1; j < matrix[0].size(); ++j)
        if (matrix[0][j] == 0)
            for (int i = 1; i < matrix.size(); ++i)
                matrix[i][j] = 0;

    if (row0 == 0)
        for (int j = 0; j < matrix[0].size(); ++j)
            matrix[0][j] = 0;

    if (col0 == 0)
        for (int i = 0; i < matrix.size(); ++i)
            matrix[i][0] = 0;
}

matrix[0][0]其实可以用来保存首列元素的信息，所以只需要额外增加首行元素的标志就行：

void setZeroes(vector<vector<int> > &matrix) {
    int row0 = 1;
    for (int j = 0; j < matrix[0].size(); ++j)
        if (matrix[0][j] == 0) {
            row0 = 0;
            break;
        }

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

    for (int i = 1; i < matrix.size(); ++i)
        if (matrix[i][0] == 0)
            for (int j = 1; j < matrix[i].size(); ++j)
                matrix[i][j] = 0;

    for (int j = 0; j < matrix[0].size(); ++j)
        if (matrix[0][j] == 0)
            for (int i = 1; i < matrix.size(); ++i)
                matrix[i][j] = 0;

    if (row0 == 0)
        for (int j = 0; j < matrix[0].size(); ++j)
            matrix[0][j] = 0;
}

LeetCode[Array]: Set Matrix Zeroes

标签：leetcode   array   matrix   算法   algorithm   

原文地址：http://blog.csdn.net/chfe007/article/details/41623245