用 set 存储 s2 的每个字符，然后遍历 s1 的每个字符，如果 set 中包含该字符，则跳过，若不包含，则输出

必须用快读和快写！！

import java.io.*;
import java.util.HashSet;
import java.util.Set;

public class Main {
    static Set<Character> set = new HashSet<>();

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter bw = new BufferedWriter(new OutputStreamWriter(System.out));
        String s1 = br.readLine();
        String s2 = br.readLine();
        for (int i = 0; i < s2.length(); i++) {
            set.add(s2.charAt(i));
        }
        for (int i = 0; i < s1.length(); i++) {
            char c = s1.charAt(i);
            if (set.contains(c)) {
                continue;
            } else {
                bw.write(c);
            }
        }
        bw.flush();
    }
}