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: ...

//참고 : https://leetcode.com/problems/matrix-block-sum/discuss/549010/C%2B%2B-oror-64.38-time-oror-100.00-space-oror-graph-illustration int r1 = max(0, i - k) + 1 ...