Add Binary

Given two binary strings, return their sum (also a binary string).

For example,
a = "11"
b = "1"

Return "100".

class Solution:    # @param a, a string    # @param b, a string    # @return a string    def addBinary(self, a, b):        length_a = len(a)        length_b = len(b)        if length_a > length_b:            b = '0' * (length_a - length_b) + b            length = length_a        else:            a = '0' * (length_b - length_a) + a            length = length_b        a = a[::-1]        b = b[::-1]        Sum = ''        carry = 0        for i in xrange(length):            tmp = ord(a[i]) - 48 + ord(b[i]) - 48 + carry            Sum += str(tmp % 2)            carry = tmp / 2        if carry == 1:            Sum += '1'        return Sum[::-1]