Leetcode 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.

click to show follow up.

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?

---

解题思路：

This problem should be solved in place, i.e., no other array should be used. We can use the first column and the first row to track if a row/column should be set to 0.

Since we used the first row and first column to mark the zero row/column, the original values are changed.

Specifically, given, the following matrix

this problem can be solved by following 4 steps:

Step 1:
First row contains zero = true;
First column contains zero = false;

Step 2: use first row and column to mark zero row and column.

Step 3: set each elements by using marks in first row and column.

Step 4: Set first column and row by using marks in step 1.

---

Java code:

public class Solution {
    public void setZeroes(int[][] matrix) {
        if(matrix == null || matrix.length == 0 || matrix[0].length == 0) {
            return;
        }
        boolean firstrowzero = false;
        boolean firstcolzero = false;
        int m = matrix.length, n = matrix[0].length;
        //set first row and column zero or not
        for(int i = 0; i < m ; i++) {
            if(matrix[i][0] == 0) {
                firstcolzero = true;
                break;
            }
        }
        for(int j = 0; j < n ; j++) {
            if(matrix[0][j] == 0) {
                firstrowzero = true;
                break;
            }
        }
        // find zeroes and store the info in first row and column
        for(int i = 1; i< m ; i++) {
            for(int j = 1; j < n; j++) {
                if(matrix[i][j] == 0) {
                    matrix[i][0] = 0;
                    matrix[0][j] = 0;
                }
            }
        }
        
        //set zeroes except the first row and column
        for(int i = 1; i < m; i++) {
            for(int j = 1; j < n; j++) {
                if(matrix[i][0] == 0 || matrix[0][j] == 0) {
                    matrix[i][j] = 0;
                }
            }
        }
        // set zeroes for first row and column if needed
        if(firstrowzero){
            for(int j = 0; j < n; j++) {
                matrix[0][j] = 0;
            }
        }
        if(firstcolzero){
            for(int i = 0; i < m; i++) {
                matrix[i][0] = 0;
            }
        }
    }
}

---

Reference:

1. http://www.programcreek.com/2012/12/leetcode-set-matrix-zeroes-java/