public Integer[] removeUsingSet(Integer[] numbers) {
    validateArray(numbers);

    Set<Integer> uniqueElements = new HashSet<Integer>();
    for (int i : numbers) {
      uniqueElements.add(i);
    }
    return uniqueElements.toArray(new Integer[uniqueElements.size()]);
  }
Implementation based on Java Sets. To solve this algorithm we have used a Java HashSet and the
complexity order of this algorithm in time is O(N) where N is the number of elements in the
input array. In space terms, the complexity order of this algorithm is also O(N), because we
are using an auxiliary data structure.

This implementation is possible in O(N) because the complexity order of HashSets for
insertions is O(1).
src/main/java/com/github/pedrovgs/problem10/RemoveDuplicates.java