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Java-based LeetCode algorithm problem solutions, regularly updated

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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:

![](https://assets.leetcode.com/uploads/2022/08/21/img.jpg)

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:**

![](https://assets.leetcode.com/uploads/2022/08/21/1.jpg)

**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:**

![](https://assets.leetcode.com/uploads/2022/08/21/2.jpg)

**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] <= 10⁶