73 Set Matrix Zeroes – Medium¶

Problem:¶

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?

Thoughts:¶

The most tricky part of this problem is to do it in place.

The trick is to leverage the first row and the first column to serve as flag.

Solutions:¶

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