class Solution {
public:
    bool isSubsequence(string s, string t) {

        unordered_map <char , vector <int>> char_idx;

        for (int i = 0 ; i < t.size(); i++)
        {
            char c = t[i];
            char_idx[c].push_back(i);
        }

        int last = -1;

        for (int i = 0 ; i < s.size(); i++)
        {
            char c = s[i];
            vector <int> vec;
           if (char_idx.find(c) != char_idx.end()) vec = char_idx[c];
            else return false;
            int first_idx = upper_bound(vec.begin(), vec.end(), last) - vec.begin();

            if (first_idx == vec.size())
                return false;
            last = vec[first_idx];
        }

        return true;

    }
};