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.

Have you met this question in a real interview? 

Yes

Example

Given a matrix

[
  [1,2],
  [0,3]
],

return
[
[0,2],
[0,0]
]

Challenge 

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?

Analysis:

 

We are going to use two sets to save all the column numbers and row numbers whose cells need to be set to 0. This will cost O(m + n) space.

 

Another approach is from: http://fisherlei.blogspot.com/2013/01/leetcode-set-matrix-zeroes.html

1.先确定第一行和第一列是否需要清零
2.扫描剩下的矩阵元素，如果遇到了0，就将对应的第一行和第一列上的元素赋值为0
3.根据第一行和第一列的信息，已经可以讲剩下的矩阵元素赋值为结果所需的值了
4.根据1中确定的状态，处理第一行和第一列。

 

 

class Solution {
    public void setZeroes(int[][] matrix) {
        if (matrix == null || matrix.length == 0 || matrix[0].length == 0) return;

        boolean firstRowZero = false, firstColumnZero = false;

        // set first row and column zero or not
        for (int i = 0; i < matrix.length; i++) {
            if (matrix[i][0] == 0) {
                firstColumnZero = true;
                break;
            }
        }

        for (int i = 0; i < matrix[0].length; i++) {
            if (matrix[0][i] == 0) {
                firstRowZero = true;
                break;
            }
        }

        // mark zeros on first row and column
        for (int i = 1; i < matrix.length; i++) {
            for (int j = 1; j < matrix[0].length; j++) {
                if (matrix[i][j] == 0) {
                    matrix[i][0] = 0;
                    matrix[0][j] = 0;
                }
            }
        }

        // use mark to set elements
        for (int i = 1; i < matrix.length; i++) {
            for (int j = 1; j < matrix[0].length; j++) {
                if (matrix[i][0] == 0 || matrix[0][j] == 0) {
                    matrix[i][j] = 0;
                }
            }
        }

        // set first column and row
        if (firstColumnZero) {
            for (int i = 0; i < matrix.length; i++)
                matrix[i][0] = 0;
        }

        if (firstRowZero) {
            for (int i = 0; i < matrix[0].length; i++)
                matrix[0][i] = 0;
        }
    }
}