import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class NationalLottery {

    public static void main(String[] args) {
        List<Integer> input = List.of(-1, 1, 3, 2, 2, 2, 5, 6, -1, 3, 6);
        List<Integer> duplicates = findDuplicates(input, 2);
        System.out.println(duplicates); // Output: [-1, 3, 6]
    }

    public static List<Integer> findDuplicates(List<Integer> input, int numberOfDuplicates) {
        if (input == null) {
            return new ArrayList<>();
        }

        Map<Integer, Integer> frequencyMap = new HashMap<>();

        for (Integer number : input) {
            if (number != null) {
                frequencyMap.put(number, frequencyMap.getOrDefault(number, 0) + 1);
            }
        }

        return frequencyMap.entrySet().stream()
                .filter(entry -> entry.getValue() == numberOfDuplicates)
                .map(Map.Entry::getKey)
                .collect(Collectors.toList());
    }
}