/** ListNode is a class used internally by the List class.  Each
   node in a List is represented as a ListNode, with an item and 
   a reference to the next node in the list.                     */

class ListNode {
  public Object item;
  public ListNode next;

  /** Postcondition: Construct a ListNode with item i and
   * next null.                                        */
  ListNode(Object obj) {
    item = obj;
    next = null;
  }


  /** Postcondition: Construct a ListNode with item i and
   * next n.                                             */
  ListNode(Object obj, ListNode n) {
    item = obj;
    next = n;
  }

    
  /** Precondition: this list is acyclic
   *  Postcondition:  Returns a reference to the node at 
   *       the given position.  If position < 1 or position > the      
   *       size number of nodes in the list, returns null.         */
  public ListNode ptrTo (int position) {
    if (position < 1) {
      return null;
    }
    else if (position == 1) {
      return this;
    }
    else if (next == null) {
      return null;
    }
    else {
      return next.ptrTo(position-1);
    }
  }
}