House Party

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

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

        int countCommands = Integer.parseInt(scanner.nextLine());

        List<String> guestName = new ArrayList<>();

        for (int count = 1; count <= countCommands; count++) {
            String command = scanner.nextLine();

            List<String> commandParts = Arrays
                    .stream(command.split(" "))
                    .collect(Collectors.toList());

            String name = commandParts.get(0);

            if (commandParts.size() == 3) {
                if (guestName.contains(name)) {
                    System.out.printf("%s is already in the list!%n", name);
                } else {
                    guestName.add(name);
                }
            } else if (commandParts.size() == 4) {
                if (guestName.contains(name)) {
                    guestName.remove(name);
                } else {
                    System.out.printf("%s is not in the list!%n", name);
                }
            }
        }
        for (String name : guestName) {
            System.out.println(name);
        }
    }
}