[C++][Add Binary Strings] short code

#1 
string Solution::addBinary(string A, string B) {
    int i = A.length() - 1, j = B.length() - 1;
    int carry = 0, len = max(i, j);
    string res = i > j ? A : B;

    while(i>=0 or j>=0) {
        if(i >= 0) carry += (A[i --] - '0');
        if(j >= 0) carry += (B[j --] - '0');
        res[len --] = (carry%2 + '0');
        carry /= 2;
    }

    return carry ? "1"+res : res;
}