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

The input strings are both non-empty and contains only characters 1 or 0.

Example 1:

Input: a = "11", b = "1"Output: "100"

Example 2:

Input: a = "1010", b = "1011"Output: "10101" 这个题我们就把a,b补全, 使得他们length相等, 这样, 只需要判断最后一个edge case, 看看是不是需要多加一位.

class Solution:    def addBinary(self, a, b):        if len(b) < len(a):            a, b = b, a        m, n, ans, pre = len(a), len(b), "", 0        a = '0'*(n-m) + a        for i in range(n)[::-1]: # note 从n-1往前走            pre, rem = divmod(int(a[i]) + int(b[i] + pre, 2))            ans += str(rem)        return ans[::-1] if not pre else '1' + ans[::-1]