leetcode 67. Add Binary

class Solution(object):
def swap(self,a,b):
a1,b1=list(a),list(b)
a1.reverse()
b1.reverse()
max_num=max(len(a1),len(b1))
if len(a1)<max_num:
a1=a1+['0']*(max_num-len(a1))
else:
b1=b1+['0']*(max_num-len(b1))
return a1,b1
def addBinary(self, aa, bb):
"""
:type a: str
:type b: str
:rtype: str
"""
ret=[]
flag=0
a,b=self.swap(aa,bb)
for i in range(len(a)):
tmp=int(a[i])+int(b[i])+flag
if tmp/2==1:
flag=1
else:
flag=0
ret.append(str(tmp%2))
if flag==1:
ret.append('1')
ret.reverse()
return  ''.join(ret)