https://leetcode.com/problems/add-binary/description/

class Solution {public:    string addBinary(string a, string b) {        string res;        int carry = 0;        for (int ia = a.length() - 1, ib = b.length() - 1; ia >= 0 || ib >= 0; ia--, ib--)        {            int ca = ia < 0 ? 0 : a[ia] - '0';            int cb = ib < 0 ? 0 : b[ib] - '0';            int c = ca + cb + carry;            res.push_back(c % 2 + '0');            carry = c / 2;        }        if (carry > 0)            res.push_back(carry + '0');        reverse(res.begin(), res.end());        return res;    }};