Leetcode 73. Set Matrix Zeroes

[medium]

Set Matrix Zeroes - LeetCode

Given an m x n integer matrix matrix, if an element is 0, set its entire row and column to 0's, and return the matrix.

You must do it in place.

Example 1:

Input: matrix = [[1,1,1],[1,0,1],[1,1,1]]
Output: [[1,0,1],[0,0,0],[1,0,1]]

Example 2:

Input: matrix = [[0,1,2,0],[3,4,5,2],[1,3,1,5]]
Output: [[0,0,0,0],[0,4,5,0],[0,3,1,0]]

Constraints:

• m == matrix.length
• n == matrix[0].length
• 1 <= m, n <= 200
• -231 <= matrix[i][j] <= 231 - 1

Follow up:

• A straightforward 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?

[Java]

1. seperate into different tasks and handle them one by one.
2. create two boolean first row, first col to see if we need to change them into 0 at the end.
3. if we find matrix[i][j] == 0, we change it first row, col location into 0, if the matrix[i][0] || matrix[0][j] are already zero, we change the boolean into true
4. use first row/ col as an index, if the index == 0, change matrix[i][j] into 0
5. if boolean is true, the the whole row or column into 0