AcWing 796. 子矩阵的和 (Prefix Sum) Describted by Java

Today train Prefix sum , two dimensional is a little complex than one dimensional,but it is not a big deal. The problem: 输入一个n行m列的整数矩阵，再输入q个询问，每一个询问包含四个整数x1, y1, x2, y2，表示一个子矩阵的左上角坐标和右下角坐标。java 对于每一个询

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