1. Thought for the Day
“Without leaps of imagination, or dreaming, we lose the excitement of possibilities. Dreaming, after all, is a form of planning.” – Gloria Steinem

2. Doubly-, Circularly-Linked List with Header Node

3. Doubly-, Circularly-Linked List with Header Node
An empty list:
head

4. Advantages No special cases
The list is never empty!
We can move freely through the list in either direction
Still cannot access elements “at random”

5. A Java Class for Deques
Use a doubly-, circularly-linked list with a header node
Class diagram:
Deque
header
addLeft, addRight, removeLeft, removeRight, rightHead, leftHead, isEmpty

6. The Deque Class public class Deque<T> { private class DequeNode
{ public T data;
public DequeNode lt, // Ptr to left node
rt; // Ptr to right node
} // inner class DequeNode
private DequeNode header; // Ptr to header
. . .
} // class Deque

7. The Deque Class: Constructor
public Deque ()
// Constructor
{ // Create header node
header = new DequeNode();
header.lt = header;
header.rt = header;
} // Constructor
header

8. The addLeft Method public void addLeft (T item)
// Add item to left end
{ DequeNode newNode = new DequeNode();
newNode.data = item;
newNode.rt = header.rt;
newNode.lt = header;
header.rt.lt = newNode;
header.rt = newNode;
} // addLeft
newNode a
header

9. Redrawing this:
header a

10. The addLeft Method (cont.)
public void addLeft (T item)
// Add item to left end
{ DequeNode newNode = new DequeNode();
newNode.data = item;
newNode.rt = header.rt;
newNode.lt = header;
header.rt.lt = newNode;
header.rt = newNode;
} // addLeft
newNode b
header a

11. Redrawing this:
header a b

12. Comments No special cases The list is never empty
public void addLeft (T item)
// Add item to left end
{ DequeNode newNode = new DequeNode();
newNode.data = item;
newNode.rt = header.rt;
newNode.lt = header;
header.rt.lt = newNode;
header.rt = newNode;
} // addLeft

13. Removing an Element public T removeLeft ()
// Remove item from left end
{ if (header.rt == header)
throw new EmptyException(…);
DequeNode tmpPtr = header.rt;
T tmpData = tmpPtr.data;
header.rt = tmpPtr.rt;
tmpPtr.rt.lt = header;
return tmpData;
} // removeLeft

14. Examining an Element and Checking for an Empty Deque
public T rightHead ()
// Return item at right end
{ if (header.lt == header)
throw new EmptyException(…);
return header.lt.data;
} // rightHead
public boolean isEmpty ()
// TRUE if no items in deque
{ return header.lt == header;
} // isEmpty

15. Doubly-, Circularly-Linked List with Header Node
Can be used for any type of list ADT
not just deques
Removes the need for special cases
Simplifies algorithms
no “previous” pointers required

16. Applications of Deques
Very general data structure
Can be used to implement stacks or queues
Problem solving
partly prioritise some possible solutions
Applications needing more flexible list handling
card games, etc.

17. Summary of List ADTs in Chapters Four and Five
List of int
Generic Lists
Stacks
Queues
Deques
ADT
IntegerVector
ArrayStack
ArrayQueue
Array
IntegerList
ObjectList
GenericList
ListStack
ListQueue
Deque
Linked List