Add Binary

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

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

Solution:

class Solution {public:    string addBinary(string a, string b) {        string res;        int lena = a.length() - 1, lenb = b.length() - 1, flag = 0;        while(lena >= 0 || lenb >= 0)        {            int sum = flag;            if(lena >= 0) sum += a[lena] - '0';            if(lenb >= 0) sum += b[lenb] - '0';            if(sum <= 1)            {                res += sum + '0';                flag = 0;            }            else            {                res += sum - 2 + '0';                flag = 1;            }            lena--;            lenb--;        }        if(flag == 1) res += '1';        reverse(res.begin(), res.end());        return res;    }};