[leetcode] 67. add binary

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

For example,
a = “11”
b = “1”
Return “100”.

class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        la =len(a)
        lb=len(b)
        if la<lb:
            return self.addBinary(b,a)
        b='0'*(la-lb)+b
        carry=0
        a=list(a)
        for i in range(la-1,-1,-1):
            sum=int(a[i])+int(b[i])+carry
            if sum==1 or sum==0:
                a[i]=str(sum)
                carry=0
            else:
                a[i]=str(sum-2)
                carry=1
        if carry==1:
            return '1'+''.join(a)
        return ''.join(a)

Status: Accepted
Runtime: 80 ms
Ranking: beats 22.46%

class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        if len(a)==0: return b
        if len(b)==0: return a
        if a[-1] == '1' and b[-1] == '1':
            return self.addBinary(self.addBinary(a[0:-1], b[0:-1]), '1') + '0'
        else:
            return self.addBinary(a[0:-1], b[0:-1]) + str(int(a[-1]) + int(b[-1]))

Status: Accepted
Runtime: 72 ms
Ranking: beats 44.74%