class Solution {
public:
    string res;
    string smallestSubsequence(string text) {
        int count[26] = {0};
        int contained[26] = {0};
        for(const auto &c : text)   ++count[c - 'a'];
        for(const auto &c : text){
            --count[c - 'a'];
            if(contained[c - 'a'])  continue;
            while(!res.empty() && c < res.back() && count[res.back() - 'a']){
                contained[res.back() - 'a'] = 0;
                res.pop_back();
            }
            res += c;
            contained[c - 'a'] = 1;
        }
        return res;
    }
};