public class Solution
extends Object
1314 - Matrix Block Sum\.
Medium
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`