Leetcode 1314. Matrix Block Sum

Given an m×n matrix and integer k, compute for each cell the sum of all elements inside the (2k+1)×(2k+1) square centered at that cell, clipped to matrix boundaries. The core challenge is computing all these sliding block sums efficiently (typically via a 2D prefix-sum / summed-area table to get each block sum in O(1) after O(mn) preprocessing).