Question

Write the method:

List <Double> duplicates(List<Double> input)

that returns a List of all duplicates in the input List. You may assume that each element occurs either once or twice. The expected runtime must be O(n) where n is the number of elements (even if the input is a LinkedList, the runtime should still be O(n). Do not change the method header.

Examples:

input = [3, 4, 4] return [4]

Answer 1

import java.util.*;

public class FindDuplicatesInList {

    List<Double> duplicates(List<Double> input) {
        List<Double> duplicates = new ArrayList<>();
        Map<Double, Integer> map = new HashMap<>();
        Iterator<Double> iterator = input.iterator();
        Double number;
        while (iterator.hasNext()) {
            number = iterator.next();
            if (map.containsKey(number)) {
                duplicates.add(number);
            } else {
                map.put(number, 0);
            }
        }
        return duplicates;
    }

    public static void main(String[] args) {
        LinkedList<Double> numbers = new LinkedList<>();
        numbers.add(3.0);
        numbers.add(4.0);
        numbers.add(4.0);

        System.out.println(new FindDuplicatesInList().duplicates(numbers));
    }
}