2373. Largest Local Values in a Matrix

Easy

You are given an n x n integer matrix grid.

Generate an integer matrix maxLocal of size (n - 2) x (n - 2) such that:

maxLocal[i][j] is equal to the largest value of the 3 x 3 matrix in grid centered around row i + 1 and column j + 1.

In other words, we want to find the largest value in every contiguous 3 x 3 matrix in grid.

Return the generated matrix.

Example 1:

Input: grid = [[9,9,8,1],[5,6,2,6],[8,2,6,4],[6,2,2,2]]
Output: [[9,9],[8,6]]
Explanation: The diagram above shows the original matrix and the generated matrix.\ Notice that each value in the generated matrix corresponds to the largest value of a contiguous 3 x 3 matrix in grid.

Example 2:

Input: grid = [[1,1,1,1,1],[1,1,1,1,1],[1,1,2,1,1],[1,1,1,1,1],[1,1,1,1,1]]
Output: [[2,2,2],[2,2,2],[2,2,2]]
Explanation: Notice that the 2 is contained within every contiguous 3 x 3 matrix in grid.

Constraints:

n == grid.length == grid[i].length
3 <= n <= 100
1 <= grid[i][j] <= 100

Solution:

class Solution {

    /**
     * @param Integer[][] $grid
     * @return Integer[][]
     */
    function largestLocal($grid) {
        $n = count($grid);
        $ans = array_fill(0, $n - 2, array_fill(0, $n - 2, 0));

        for ($i = 0; $i < $n - 2; ++$i){
            for ($j = 0; $j < $n - 2; ++$j){
                for ($x = $i; $x < $i + 3; ++$x){
                    for ($y = $j; $y < $j + 3; ++$y){
                        $ans[$i][$j] = max($ans[$i][$j], $grid[$x][$y]);
                    }
                }
            }
        }

        return $ans;
    }
}

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