Sum of Two Integers

LeetCode 371

class Solution:
    def getSum(self, a, b):
        """
        :type a: int
        :type b: int
        :rtype: int
        """
        mask = 0xFFFFFFFF
        maxi = 0x7FFFFFFF
        """
        011  3
        010  2
        001  1
        000  0
        111 -1
        110 -2
        101 -3
        """
        if b ==0 and a <= maxi:
            return a
        elif b == 0 and a > maxi:
            return ~(a ^ mask)
        else:
            s = (a ^ b) & mask
            c = ((a & b) << 1) & mask
            return self.getSum(s, c)