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DIY: Range Sum Query 2D — Immutable

Explore how to build a NumMatrix class in Kotlin that efficiently calculates the sum of sub-regions in a 2D matrix. Understand the matrix constraints, how to handle multiple sum queries, and implement the sumRegion method to solve range sum problems in coding interviews.

Problem statement

Given an m * n matrix, you need to handle multiple queries of the following type:

Calculate the sum of the elements of the matrix inside the rectangle defined by its upper left corner, (row1,col1)(row1, col1) ...