67. Add Binary

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;
    }
};