Maximum Sum of an Hourglass - LeetCode solutions

Problem

You are given an m x n integer matrix grid.

We define an hourglass as a part of the matrix with the following form:

Return **the *maximum* sum of the elements of an hourglass**.

Note that an hourglass cannot be rotated and must be entirely contained within the matrix.

Example 1:

Input: grid = [[6,2,1,3],[4,2,1,5],[9,2,8,7],[4,1,2,9]]
Output: 30
Explanation: The cells shown above represent the hourglass with the maximum sum: 6 + 2 + 1 + 2 + 9 + 2 + 8 = 30.

Example 2:

Input: grid = [[1,2,3],[4,5,6],[7,8,9]]
Output: 35
Explanation: There is only one hourglass in the matrix, with the sum: 1 + 2 + 3 + 5 + 7 + 8 + 9 = 35.

Constraints:

m == grid.length
n == grid[i].length
3 <= m, n <= 150
0 <= grid[i][j] <= 10^6

Solution (Java)

class Solution {
    public int maxSum(int[][] grid) {

        int r = grid.length;
        int c = grid[0].length;

        int max = 0;

        for(int i = 0; i < r-2; i++){
            for(int j = 0; j < c-2; j++){

                int sum = 0;
                for(int v = i; v < i+3; v++){
                    for(int k = j; k < j+3; k++){
                        sum += grid[v][k];
                    }
                }

                sum = sum - grid[i+1][j] - grid[i+1][j+2];

                max = Math.max(max, sum);
            }
        }

        return max;
    }
}

Explain:

nope.

Complexity:

Time complexity : O(n).
Space complexity : O(n).