LeetCode:Set Matrix Zeroes

Set Matrix Zeroes

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Total Accepted: 65397  Total Submissions: 195523  Difficulty: Medium

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

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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?

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思路：

例：matrix[m][n]
0 1 2 5
4 3 1 4
0 1 0 4
1 2 1 3
2 0 1 1

（1）先根据matrix中的元素，把首行、首列的元素置0

0 0 0 5
4 3 1 4
0 1 0 4
1 2 1 3
0 0 1 1

（2）根据首先、首列是否为0，设置row在[1,m-1],col在[1,n-1]各元素是否为0
0 0 0 5
4 0 0 4
0 0 0 0
1 0 0 3
0 0 0 0

（3）最后处理首行、首列
0 0 0 0
0 0 0 4
0 0 0 0
0 0 0 3
0 0 0 0

java code:

public class Solution {
    public void setZeroes(int[][] matrix) {
        
        boolean firstRow = false, firstCol = false;
        
        int rows = matrix.length;
        int cols = matrix[0].length;
        
        for(int i=0;i<rows;i++) {
            for(int j=0;j<cols;j++) {
                if(matrix[i][j]==0){
                    if(i==0) firstRow = true;
                    if(j==0) firstCol = true;
                    
                    matrix[i][0]=matrix[0][j] = 0;
                }
            }
        }
        
        for(int i=1;i<rows;i++) {
            for(int j=1;j<cols;j++) {
                if(matrix[i][0]==0 || matrix[0][j]==0) matrix[i][j] = 0;
            }
        }
        
        if(firstRow)
            for(int i=0;i<cols;i++) matrix[0][i] = 0;
        
        if(firstCol)
            for(int j=0;j<rows;j++) matrix[j][0] = 0;
        
    }
}