0073. Set Matrix Zeroes

https://leetcode.com/problems/set-matrix-zeroes

Description

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?

ac

the key is be careful about first row and column

class Solution {
    public void setZeroes(int[][] matrix) {
        int mR = matrix.length;
        int mC = matrix[0].length;
        // check first row, first column
        boolean setFirstRow = false;
        boolean setFirstCol = false;
        for (int c = 0; c < mC; c++) {
            if (matrix[0][c] == 0) setFirstRow = true;
        }
        for (int r = 0; r < mR; r++) {
            if (matrix[r][0] == 0) setFirstCol = true;
        }
        // check the rest, mark it first row/col
        for (int r = 1; r < mR; r++) {
            for (int c = 1; c < mC; c++) {
                if (matrix[r][c] == 0) {
                    matrix[0][c] = 0;
                    matrix[r][0] = 0;
                }
            }
        }
        // set row and col
        for (int c = 1; c < mC; c++) {
            if (matrix[0][c] == 0) {
                for (int r = 0; r < mR; r++) matrix[r][c] = 0;
            }
        }
        for (int r = 1; r < mR; r++) {
            if (matrix[r][0] == 0){
                for (int c = 0; c < mC; c++) matrix[r][c] = 0;
            }
        }
        // set first row/col
        if (setFirstRow) {
            for (int c = 0; c < mC; c++) matrix[0][c] = 0;
        }
        if (setFirstCol) {
            for (int r = 0; r < mR; r++) matrix[r][0] = 0;
        }
    }
}