leetcode 304. Range Sum Query 2D

class NumMatrix(object):    def __init__(self, matrix):        """        :type matrix: List[List[int]]        """        if not matrix:            self.table = None            return        self.table = [ [ 0 for i in range(len(matrix[0])+1) ] for j in range(len(matrix)+1) ]        self.table[0][0] = matrix[0][0]        for i in range(1,len(matrix)+1,1):            for j in range(1,len(matrix[0])+1,1):                self.table[i][j] = self.table[i-1][j] + self.table[i][j-1]-self.table[i-1][j-1] + matrix[i-1][j-1]    def sumRegion(self, row1, col1, row2, col2):        """        :type row1: int        :type col1: int        :type row2: int        :type col2: int        :rtype: int        """        if not self.table:            return None        else:              return self.table[row2+1][col2+1] + self.table[row1][col1] -\                    self.table[row2+1][col1] - self.table[row1][col2+1]