1081

class Solution {
public:
    string smallestSubsequence(string s) {
        string ans =  "";
        int last[26] = {0};
        int seen[26] = {0};

        for(int i=0; i<s.length(); i++){
            last[s[i]-'a'] = i;
        }

        for(int i=0; i<s.length(); i++){
            if(seen[s[i]-'a']) continue;
            while(!ans.empty() && ans.back() > s[i] && last[ans.back()-'a'] > i){
                seen[ans.back()-'a']--;
                ans.pop_back();
            }
            ans.push_back(s[i]);
            seen[s[i]-'a']++;
        }
        return ans;
    }
};