【leetcode】67. Add Binary

题目如下：

解题思路：首先将较短的输入前面补0，保证a和b长度一样，然后逐位相加再加上进位的值。如果和为3，当前位值为1，进位1；如果为2，当前位值为0，进位为1；否则不进位，当前位值即为和的值。

代码如下：

class Solution(object):
    def addBinary(self, a, b):
        """
        :type a: str
        :type b: str
        :rtype: str
        """
        diff = abs(len(a) - len(b))
        if len(a) < len(b):
            a = '0'*diff + a
        elif len(b) < len(a):
            b= '0'*diff + b
        carry = 0
        res = ''
        for i,j in zip(a[::-1],b[::-1]):
            v = int(i) + int(j) + carry
            if v == 3:
                v = 1
                carry = 1
            elif v == 2:
                v = 0
                carry = 1
            else:
                carry = 0
            res = str(v) + res
        if carry > 0:
            res = str(carry) + res
        return res