My recursive C++ solution

string _str( string & s, string & s1, int n1, int n2)
{
    if (n1 == 0 || n2 == 0)
    {   
        if( s[0]== s1[0] ) return "YES";

        return "NO";
    }
    if (s[n1] == s1[n2])
        return "YES";

    return _str(s, s1, n1 - 1, n2 - 1);
}

string twoStrings(string s1, string s2) 
{
    return _str(s1, s2, s1.size() - 1, s2.size() - 1);
}