304. Range Sum Query 2D - Immutable

304. Range Sum Query 2D - Immutable

解題方式同： 303. Range Sum Query - Immutable

class NumMatrix:

    def __init__(self, matrix: List[List[int]]):
        rows = len(matrix)
        cols = len(matrix[0])
        if rows == 0 or cols == 0:
            return
        self.preSum = [[0 for _ in range(cols + 1)] for _ in range(rows + 1)]
        for i in range(1, rows + 1):
            for j in range(1, cols + 1):
                self.preSum[i][j] = self.preSum[i-1][j] + self.preSum[i][j-1] + matrix[i-1][j-1] - self.preSum[i-1][j-1]


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