LeetCode-in-All

73. Set Matrix Zeroes

Medium

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?

Solution

class Solution {
    /**
     * @param Integer[][] $matrix
     * @return NULL
     */
    public function setZeroes(&$matrix) {
        $m = count($matrix);
        $n = count($matrix[0]);
        $row0 = false;
        $col0 = false;
        // Check if 0th col needs to be market all 0s in future
        foreach ($matrix as $ints) {
            if ($ints[0] == 0) {
                $col0 = true;
                break;
            }
        }
        // Check if 0th row needs to be market all 0s in future
        for ($i = 0; $i < $n; $i++) {
            if ($matrix[0][$i] == 0) {
                $row0 = true;
                break;
            }
        }
        // Store the signals in 0th row and column
        for ($i = 1; $i < $m; $i++) {
            for ($j = 1; $j < $n; $j++) {
                if ($matrix[$i][$j] == 0) {
                    $matrix[$i][0] = 0;
                    $matrix[0][$j] = 0;
                }
            }
        }
        // Mark 0 for all cells based on signal from 0th row and 0th column
        for ($i = 1; $i < $m; $i++) {
            for ($j = 1; $j < $n; $j++) {
                if ($matrix[$i][0] == 0 || $matrix[0][$j] == 0) {
                    $matrix[$i][$j] = 0;
                }
            }
        }
        // Set 0th column
        for ($i = 0; $i < $m; $i++) {
            if ($col0) {
                $matrix[$i][0] = 0;
            }
        }
        // Set 0th row
        for ($i = 0; $i < $n; $i++) {
            if ($row0) {
                $matrix[0][$i] = 0;
            }
        }
    }
}

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