1. Doubly Linked Lists
Lecture 21
Tue, Mar 21, 2006

2. Topics Doubly Linked Lists Doubly Linked List Nodes
Validity Requirements
Inserting and Deleting

3. Doubly Linked Lists
A doubly linked list is a linked list in which each node has two links
A forward pointer to its successor
A backwards pointer to its predecessor.

4. Doubly Linked List Nodes
A DoublyLinkedListNode has three data members:
T data - Value stored in the node.
DoublyLinkedListNode* prev - Pointer to the previous node.
DoublyLinkedListNode* next - Pointer to the next node.

5. The Data Members A DoublyLinkedList object has three data members:
int size - Number of elements in the list.
DoublyLinkedListNode* head - Pointer to the first node.
DoublyLinkedListNode* tail - Pointer to the last node.

6. Chasing Pointers We can move both forwards and backwards in the list.
When chasing pointers, it is not necessary to keep a pointer to the previous node.

7. Validity Requirements
size >= 0.
If size == 0, then
head == NULL
tail == NULL
If size >= 1, then
head != NULL
tail != NULL

8. Validity Requirements
If size == 1, then
head == tail
If size >= 1, then
The next pointer in node size is NULL.
The prev pointer in node 1 is NULL.

9. Validity Requirements
If size >= 2, then
For every i from 1 to size - 1, the next pointer in node i is not NULL.
For every i from 2 to size, the prev pointer in node i is not NULL.
The prev pointer of node 2 equals head.
The next pointer of node size - 1 equals tail.

10. Validity Requirements
If size >= 3, then
For every i from 2 to size - 1, the next pointer of node i - 1 equals the prev pointer of node i + 1.

11. Inserting into a Doubly Linked List
Locate the position for the insertion.
Create the new node.
Update the forward pointers.
Update the backward pointers.
Increment the size of the list.
Do we need both the prev and curr pointers?

12. Deleting from a Doubly Linked List
Locate the position for the deletion.
Update the forward pointer.
Update the backward pointer.
Delete the node.
Decrement the size of the list.

13. The DoublyLinkedList Class
Example
doublylinkedlistnode.h
doublylinkedlist.h
ListTest.cpp