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

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

题目解析：

对string了解不熟练，本来想着一直在res前加字符，但不会用，后来就在res后加字符然后调用reverse翻转。其实用:res = (char)(tmp+’0′) + res;即可。

class Solution {
public:
    string addBinary(string a, string b) {
        int alen = a.size();
        int blen = b.size();
        if(!alen)
            return b;
        if(!blen)
            return a;

        int carry  = 0;
        string res;
        int i,j;
        //从后向前依次求和
        for(i = alen-1,j = blen-1;i>=0 && j>=0;i--,j--){
            int tmp = a[i]-'0'+b[j]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
        }
        //如果j没遍历完，就接着遍历
        while(j>=0){
            int tmp = b[j]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
            j--;
        }

        while(i>=0){
            int tmp = a[i]-'0'+carry;
            if(tmp >= 2)
                carry = 1;
            else
                carry = 0;
            res += tmp%2+'0';
            i--;
        }
        //处理最后还有进位的情况
        if(carry)
            res+='1';
        reverse(res.begin(),res.end());
        return res;
    }
};