Walk a Doubly Linked List Both Ways

Every DNode carries a prev as well as a next, so the same chain can be read from either end. addFirst() and addLast() already keep both links wired.

Requirements:
1. printForward() returns one string built from head, following next.
2. printBackward() returns one string built from tail, following prev.
3. Both join the values with an arrow and finish with the word null, so an empty list gives just that word.

The DNode class is provided - do not rename it and do not add public to it, because a Java source file may contain only one public class.

Do NOT write a main method.