Add Binary

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

For example,
a = "11"
b = "1"
Return "100".

分析：该题需要注意的地方：1）两个string的长度可能不同 2）近位 

借鉴leetcode-cpp.pdf的做法，先将两个string reverse，这样可以很方便的处理低位对齐的问题。代码如下：

class Solution {
public:
    string addBinary(string a, string b) {
        string res;
        size_t n = a.size()>b.size()?a.size():b.size();
        reverse(a.begin(),a.end());
        reverse(b.begin(),b.end());
        int carry = 0;
        for(int i = 0; i < n; i++){
            int ai = i<a.size()?a[i]-'0':0;
            int bi = i<b.size()?b[i]-'0':0;
            int val = (ai+bi+carry)%2;
            carry = (ai+bi+carry)/2;
            res.insert(res.begin(),val+'0');
        }
        if(carry == 1)
            res.insert(res.begin(),'1');
        return res;
    }
};