class Solution {

public:

    string removeDuplicateLetters(string s) {

        int size = s.size();

        if (size == 0) return "";

        int l = 0;

        string ret = "";

        vector<int> cnt(26, 0);

        for (int j = 0; j < size; ++j) {

            cnt[s[j] - 'a'] = 1;

        }

        int total_diff = std::accumulate(cnt.begin(), cnt.end(), 0);

        for (int z = 0; z < total_diff; ++z) {

            for (int i = 0; i < 26; ++i) {

                int appear = -1;

                for (int j = l; j < size; ++j) {

                    if (s[j] - 'a' == i && ret.find('a' + i) == -1) {

                        appear = j;

                        break;

                    }

                }

                if (appear == -1) continue;

                vector<int> cnt2(26, 0);

                for (int j = appear; j < size; ++j)

                    cnt2[s[j] - 'a'] = 1;

                for (auto c : ret)

                    cnt2[c - 'a'] = 1;

                int num = std::accumulate(cnt2.begin(), cnt2.end(), 0);

                if (num == total_diff) {

                    ret += char('a' + i);

                    l = appear + 1;

                    break;

                }

            }

        }

        return ret;

    }

};

class Solution {

public:

    string removeDuplicateLetters(string s) {

        vector<int> cand(256, 0);

        vector<bool> visited(256, false);

        for (auto c : s)

            cand[c]++;

        string ret = "0";

        for (auto c : s) {

            cand[c]--;

            if (visited[c]) continue;

            while (c < ret.back() && cand[ret.back()]) {

                visited[ret.back()] = false;

                ret.pop_back();

            }

            visited[c] = true;

            ret += c;

        }

        return ret.substr(1);

    }

};