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##秋雨绵绵顾自怜 独坐室中狂刷题

##管他日日笙箫起 我自地里刨根底
leetcode304. Range Sum Query 2D- Immutable

Example:
Given matrix = [
[3, 0, 1, 4, 2],
[5, 6, 3, 2, 1],
[1, 2, 0, 1, 5],
[4, 1, 0, 1, 7],
[1, 0, 3, 0, 5]
]

sumRegion(2, 1, 4, 3) -> 8
sumRegion(1, 1, 2, 2) -> 11
sumRegion(1, 2, 2, 4) -> 12

dp solution: