提取不重复的数字

import java.io.*;
import java.util.*;

public class Main{
    public static void main(String[] args) throws Exception{
        Scanner sc = new Scanner(System.in);
        while(sc.hasNextLine()){
            String s = sc.nextLine();
            HashSet<Character> set = new HashSet<>();
            ArrayList<Character> list = new ArrayList<>();
            for(int i = s.length() - 1; i >= 0; --i){
                if(!set.contains(s.charAt(i))){
                    set.add(s.charAt(i));
                    list.add(s.charAt(i));
                }
            }
            for(int i = 0; i < list.size(); ++i){
                System.out.print(list.get(i));
            }
                System.out.println();
        }
    }
}