67. Add Binary

class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        if len(a) < len(b):
            return self.addBinary(b, a)
        b = '0' * (len(a) - len(b)) + b
        temp = 0
        res = ''
        for i in range(len(a)-1, -1, -1):
            ans = int(a[i]) + int(b[i]) + temp
            if ans == 0:
                res = '0' + res
                temp = 0
            elif ans == 1:
                res = '1' + res
                temp = 0
            elif ans == 2:
                res = '0' + res
                temp = 1
            else:
                res = '1' + res
                temp = 1
        if temp == 1:
            res = '1' + res
        return res