CS 261 - Winter 2010

Linked Lists - Part 2

Deque with Double Links and Sentinel, Bag

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

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;};

Picture of List with double link

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

Picture of List with Sentinel

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++)

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;

};

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

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.

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); }

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

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

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.

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);

}

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?

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

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)

Now your chance

• Questions?

• Doubly linked list is used in programming assignment 3. Mainly you need to write addLink and removeLink

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 … )