每日一题力扣304 二维的前缀和

给定一个二维矩阵，计算其子矩形范围内元素的总和，该子矩阵的左上角为 (row1, col1) ，右下角为 (row2, col2) 。

正解：

class NumMatrix:

    def __init__(self, matrix: List[List[int]]):
        if not matrix or not matrix[0]:
            M, N = 0, 0
        else:
            M, N = len(matrix), len(matrix[0])
        self.preSum = [[0] * (N + 1) for _ in range(M + 1)]#构造一个0矩阵

        for i in range(M):
            for j in range(N):#presum是00到ij的和
                self.preSum[i + 1][j + 1] = self.preSum[i][j+1]+self.preSum[i+1][j]-self.preSum[i][j]+matrix[i][j]


    def sumRegion(self, row1: int, col1: int, row2: int, col2: int) -> int:
        return self.preSum[row2+1][col2+1]-self.preSum[row2+1][col1]-self.preSum[row1][col2+1]+self.preSum[row1][col1]