2428 - Maximum Sum of an Hourglass.

Medium

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

• <code>0 <= gridj<= 10⁶</code>