class Solution {public:    string addBinary(string a, string b) {        int carry = 0, alen = a.length(),blen = b.length();        string res="";        if(alen > blen) {b=string(alen-blen,‘0‘)+b;blen = alen;}        if(alen < blen) {a=string(blen-alen,‘0‘)+a;alen = blen;}        for(int i = alen-1; i>=0 ; -- i){            int num =(a[i]-‘0‘)+(b[i]-‘0‘)+carry;            carry = 0;            if(num >= 2) {num-=2;carry=1;}            res+=‘0‘+num;        }        if(carry) res+=‘1‘;        reverse(res.begin(),res.end());        return res;    }};