private static class Node {
    E item;
    Node next;
    Node prev;
    Node(Node prev, E element, Node next) {
      this.item = element;
      this.next = next;
      this.prev = prev;
    }
  }

  transient int size = 0;
  /**
   * Pointer to first node.
   * Invariant: (first == null && last == null) ||
   *      (first.prev == null && first.item != null)
   **/
  transient Node first;
  /**
   * Pointer to last node.
   * Invariant: (first == null && last == null) ||
   *      (last.next == null && last.item != null)
   **/
  transient Node last;

  /**
   * Returns the element at the specified position in this list.
   * @param index index of the element to return
   * @return the element at the specified position in this list
   * @throws IndexOutOfBoundsException {@inheritDoc}
   **/
  public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
  }
  /**
   * Returns the (non-null) Node at the specified element index.
   **/
  Node node(int index) {
    // assert isElementIndex(index);
    if (index < (size >> 1)) {
      Node x = first;
      for (int i = 0; i < index; i++)
        x = x.next;
      return x;
    } else {
      Node x = last;
      for (int i = size - 1; i > index; i--)
        x = x.prev;
      return x;
    }
  }

  /**
   * Appends the specified element to the end of this list.
   * 
This method is equivalent to {@link #addLast}.
   * @param e element to be appended to this list
   * @return {@code true} (as specified by {@link Collection#add})
   **/
  public boolean add(E e) {
    linkLast(e);
    return true;
  }
  /**
   * Links e as last element.
   **/
  void linkLast(E e) {
    final Node l = last;
    final Node newNode = new Node<>(l, e, null);
    last = newNode;
    if (l == null)
      first = newNode;
    else
      l.next = newNode;
    size++;
    modCount++;
  }