StringBuffer的思路

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while (sc.hasNext()) {
            String input = sc.nextLine();
            StringBuffer sb = new StringBuffer(input).reverse();
            boolean[] exist = new boolean[10]; // 用 boolean 数组缓存，初始化全false

            for(int i = 0;i < sb.length(); i++) {
                if (exist[sb.charAt(i) - '0']) {
                    sb.deleteCharAt(i);
                    i--; // 删除后要指回同一位置
                } else {
                    exist[sb.charAt(i) - '0'] = true;
                }
            }

            System.out.println(sb.toString());
        }
    }
}