This was the add method we wrote in class without using the dummy nodes
at the front and back of the list.  The complexity of this code is similar
to what you might see on the final, but the final exam will not have a back
field to worry about and it won't be a doubly-linked list (with next and
prev references), it will only be singly-linked (with next references).

When I tried to write this code, I made mistakes until I took it slowly
and drew pictures.  Please draw pictures when you study and take the exam!


      ListNode newNode;
      if (index == 0) {
         newNode = new ListNode(value, front, null);
         front = newNode;
         if (newNode.next != null) {
            newNode.next.prev = newNode;
         }
      } else {
         ListNode current = nodeAt(index - 1);
         newNode = new ListNode(value, current.next, current);
         current.next = newNode;
         if (newNode.next != null) {
            newNode.next.prev = newNode;
         }
      }
      if (index == size) {
         back = newNode;
      }
      size++