한 번만 등장한 문자(프로그래머스)

 

import java.util.*;
class Solution {
    public String solution(String s) {
        String answer ="";
        char [] carr = s.toCharArray();
        Hashtable<Character , Integer> hs = new Hashtable<>();
        for (char c : carr){
            hs.put(c, hs.getOrDefault(c, 0) + 1);
        }
        List<Character> list = new ArrayList(hs.keySet());
        Collections.sort(list);
       for (Character key : list){
           if(hs.get(key) == 1){
               answer += key;
           }
       }
        return answer;
    }
}