CS 261 - Winter 2010

1. CS 261 - Winter 2010 Linked Lists - Part 2 Deque with Double Links and Sentinel, Bag

2. Three New Variations • Today we are going to continue to explore Linked Lists with three new variations • Double links - links both forwards and backwards • Sentinel - a Special marker link at end • Keep size as part of the structure • Close to the way standard libs do it

3. Double Links • Double links point both forwards and backwards. Allow access to both next and previous link struct link { EleType value; struct link * next; struct link * previous; };

4. Picture of List with double link

5. 2nd Variation: Sentinels • A Sentinel is a special marker node at front or back • Has no value, is never removed • Can remove special case code, since pointer is never null • An “Empty” list still has a sentinel node

6. Picture of List with Sentinel

7. If one Sentinel is good • Just to make the LinkedList slightly more interesting, we will add a sentinel to BOTH front and back • Eliminates even more special cases • most “Real” LinkedList use two sentinels, double links, and more or less combines all the interfaces. (Java, C++)

8. Keep size as a field in the header • Maintain the size value explicitly in header. Can you think why we want to do it this way? struct linkedList { struct link * frontSentinel; struct link * backSentinel; int size; };

9. Deque • Why would you use a sentinel? • Consider a deque, with pointer to front and pointer to sentinel • How do you access the front of the queue? How do you access the back? • Draw pictures, name the values you are looking for

10. Generalize add • What about adding values • Remember, for a queue, can add either to the front OR to the back • Add to front and add to back are now special cases of more general “add After” operation.

11. Deque code void listDequeAddBack (struct list *q, EleType newValue) { _addLinkAfter(q, q->frontSentinel, newValue); } void listDequeAddFront (struct list *q, EleType newValue) { _addLinkAfter(q, q->backSentinel->prev, newValue); }

12. Are there any special cases? • Draw pictures • Does this work even if the list is empty? • Always try to imagine special cases, make sure your code works for them

13. Both a stack and a queue • To use a deque as a stack, use addFront and removeFront • To use as a queue, use addBack and removeFront (or vice versa) • Works for both equally well

14. What does add do? • Needs to add a new link to a chain, right before the link given as the argument. • Draw a picture. Label the names of everything you know. Think through the steps. • Why are we passing pointer to header? • You will get to do this in the worksheet.

15. Removes also generalized void listRemoveFirst (struct list *q) { assert(! listIsEmpty(q)); _removeLink (q, q->firstLink); } void listRemoveLast (struct list *q) { assert(! listIsEmpty(q)); _removeLink (q, q->sentinel->prev); }

16. Again, draw pictures • Draw a picture to see what you are doing. Label the names for things. • What are the steps? Do they need to be done in a particular order? • Why are we passing the header node to the removeLink routine?

17. You will get your chance • You will get your chance to write addLink and removeLink in a moment. • The same removeLink idea can be used to implement the remove operation in the LinkedListBag • Remember from yesterday, remove was the only complicated operation

18. Remember the Bag operations • Add (already done) • Contains (same as before, walk down the links, looking at every element) • Remove (walk down the links, when you find the value you want, call removeLink) • Size (this one is easy)

19. Now your chance • Questions? • Doubly linked list is used in programming assignment 3. Mainly you need to write addLink and removeLink

20. Oh, that exam on Tuesday? • Questions on big-Oh (what is the running time of …) • Implementation questions like we have been doing (how to implement … )