有序Set：LinkedHashSet

import java.util.Scanner;
import java.util.LinkedHashSet;
// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        char[] str = "abcdefghijklmnopqrstuvwxyz".toCharArray();
        Scanner in = new Scanner(System.in);
        String s1 = in.nextLine();
        char[] c1 = s1.toCharArray();
        String s2 = in.nextLine();
        StringBuilder sb = new StringBuilder();
        LinkedHashSet<Character> s = new LinkedHashSet<>();
        for (int i = 0; i < c1.length; i++) {
            s.add(c1[i]);
        }
        for (int i = 0; i < str.length; i++) {
            s.add(str[i]);
        }
        Object[] array = s.toArray();
        for (int i = 0; i < s2.length(); i++) {
            int n = s2.charAt(i) - 'a';
            sb.append(array[n]);
        }
        System.out.println(sb);
    }
}

LinkedHashSet<Character> s = new LinkedHashSet<>();
Object[] array = s.toArray();