public class test<E> {
List<E> input = new ArrayList<>();
 public test(List<E> input) {
  this.input = input
 // modify this constructor if necessary, but do not remove default constructor
 }
 // add other constructors if necessary
 /**
 * Returns the queue that adds an item into the tail of this queue without modifying this queue.
 * <pre>
 * e.g.
 * When this queue represents the queue (2, 1, 2, 2, 6) and we enqueue the value 4 into this queue,
 * this method returns a new queue (2, 1, 2, 2, 6, 4)
 * and this object still represents the queue (2, 1, 2, 2, 6) .
 * </pre>
 * If the element e is null, throws IllegalArgumentException.
 * @param e
 * @return
 * @throws IllegalArgumentException
 */
 public test<E> enqueue(E e) {
 return null;
 }