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.

链接： http://leetcode.com/problems/set-matrix-zeroes/

5/28/2017

3ms, 17%

先确定第一行和第一列是否应该全部置为0，用firstRow, firstColumn表示。这里不改数组的原因是这一行一列会被用作其他行列的标志位。

遍历其他行列，并将第一行第一列相应元素设为0

遍历第一行第一列将相应元素设为0

public class Solution {
    public void setZeroes(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return;
        }
        int row = matrix.length;
        int column = matrix[0].length;
        boolean firstRow = false, firstColumn = false;
        
        if (!firstColumn) {
            for (int i = 0; i < row; i++) {
                if (matrix[i][0] == 0) {
                    firstColumn = true;
                    break;
                }
            }
        }
        if (!firstRow) {
            for (int i = 0; i < column; i++) {
                if (matrix[0][i] == 0) {
                    firstRow = true;
                    break;
                }
            }
        }        
        for (int i = 1; i < row; i++) {
            for (int j = 1; j < column; j++) {
                if (matrix[i][j] == 0) {
                    matrix[0][j] = 0;
                    matrix[i][0] = 0;
                }
            }
        }
        for (int i = 1; i < row; i++) {
            if (matrix[i][0] == 0) {
                for (int j = 1; j < column; j++) {
                    matrix[i][j] = 0;
                }
            }
        }
        for (int i = 1; i < column; i++) {
            if (matrix[0][i] == 0) {
                for (int j = 1; j < row; j++) {
                    matrix[j][i] = 0;
                }
            }
        }
        if (firstColumn) {
            for (int i = 0; i < row; i++) {
                matrix[i][0] = 0;
            }
        }
        if (firstRow) {
            for (int i = 0; i < column; i++) {
                matrix[0][i] = 0;
            }
        }
        return;
    }
}

别人的算法，大部分跟我写的一样。有一个特别精简的：设第一行第一列用top-down，设其他元素用bottom-up

https://discuss.leetcode.com/topic/5056/any-shorter-o-1-space-solution

void setZeroes(vector<vector<int> > &matrix) {
    int col0 = 1, rows = matrix.size(), cols = matrix[0].size();

    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;
    }
}

更多讨论

https://discuss.leetcode.com/category/81/set-matrix-zeroes