Number_of_Matching_Subsequences

        vector<vector<int>> alpha (26);
        for (int i = 0; i < S.size (); ++i) alpha[S[i] - 'a'].push_back (i);
        int res = 0;
        //ace
        for (const auto& word : words) {
            int x = -1;
            bool found = true;

            for (char c : word) {
                auto it = upper_bound (alpha[c - 'a'].begin (), alpha[c - 'a'].end (), x);
                if (it == alpha[c - 'a'].end ()) found = false;
                else x = *it;
            }

            if (found) res++;
        }

        return res;
    }