1314. Matrix Block Sum 
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Problem
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.lengthn == mat[i].length1 <= m, n, k <= 1001 <= mat[i][j] <= 100