67. Add Binary

问题描述：

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"

解题思路：

我原来的代码巨长无比（就是正常的思路）

想看看有没有人又简短的代码。

我就知道，一定有。

参考：Short code by c++

代码：

class Solution {
public:
    string addBinary(string a, string b) {
        int len1 = a.size();
        if(len1 == 0) return b;
        int len2 = b.size();
        if(len2 == 0) return a;
        
        int c = 0, i = len1-1, j = len2-1;
        string s;
        while(c > 0 || i > -1 || j > -1){
            c += i > -1 ? a[i--] - '0' : 0;
            c += j > -1 ? b[j--] - '0' : 0;
            s.push_back(char(c%2 + '0'));
            c =  c/2;
        }
        reverse(s.begin(), s.end());
        return s;
    }
};