SlowestSolutionInTheHistory?

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Scanner;
import java.util.stream.Collectors;

public class test {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        List<Integer> arr = Arrays.stream(scanner.nextLine().split(",")).map(Integer::parseInt).collect(Collectors.toList());
        List<String> result = new ArrayList<>();

        for (int i = 1; i <= arr.size(); i++) {
            if (!arr.contains(i)) {
                result.add(String.valueOf(i));
            }
        }
        System.out.println(String.join(",", result));
    }
}