Python, LintCode, 1. A + B 问题

class Solution:
    """
    @param a: An integer
    @param b: An integer
    @return: The sum of a and b 
    """
    def aplusb(self, a, b):
        # write your code here
        carry = 0
        val = 0
        sum = 0
        for i in range(32):
            xa = a & 1
            xb = b & 1
            if (xa==0) & (xb==0):
                val = carry
                carry = 0
            elif (xa==1) & (xb==1):
                val = carry
                carry = 1
            elif carry==1:
                val = 0
                carry = 1
            else:
                val = 1
                carry = 0
            a = a>>1
            b = b>>1
            val = val<