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# 1314. Matrix Block Sum

<https://leetcode.com/problems/matrix-block-sum>

## Description

Given a `m x n` matrix `mat` and an integer `k`, return *a matrix* `answer` *where each* `answer[i][j]` *is the sum of all elements* `mat[r][c]` *for*:

* `i - k <= r <= i + k,`
* `j - k <= c <= j + k`, and
* `(r, c)` is a valid position in the matrix.

**Example 1:**

```
**Input:** mat = [[1,2,3],[4,5,6],[7,8,9]], k = 1
**Output:** [[12,21,16],[27,45,33],[24,39,28]]
```

**Example 2:**

```
**Input:** mat = [[1,2,3],[4,5,6],[7,8,9]], k = 2
**Output:** [[45,45,45],[45,45,45],[45,45,45]]
```

**Constraints:**

* `m == mat.length`
* `n == mat[i].length`
* `1 <= m, n, k <= 100`
* `1 <= mat[i][j] <= 100`

## ac

```java
```