class Solution {

public:

    string addBinary(string a, string b) {

        int carry = , alen = a.length(),blen = b.length();

        string res="";

        if(alen > blen) {b=string(alen-blen,'')+b;blen = alen;}

        if(alen < blen) {a=string(blen-alen,'')+a;alen = blen;}

        for(int i = alen-; i>= ; -- i){

            int num =(a[i]-'')+(b[i]-'')+carry;

            carry = ;

            if(num >= ) {num-=;carry=;}

            res+=''+num;

        }

        if(carry) res+='';

        reverse(res.begin(),res.end());

        return res;

    }

};