Linked List

Linked List Joe Meehean

Linked List … item N phead: item 1 • Conceptual Picture • N items • chained together using pointers • pointed to by variable head • Advantage • allows list to grow indefinitely without copying • constant time inserts and removes • just (un)hook into/from chain

List Node item 1 • Private nested class • nested class: class defined within a class • private nested: not visible outside outer class • Member variables • public: • T data; • Node<T> *next; • Constructor • Node( const T& data = T(), Node<T>* n = NULL)

Connecting List Nodes Node<string>* head; phead: head = new Node<string>(“ant”); ant phead: head->next = new Node<string>(“bat”); bat ant phead:

DISCUSSION BREAK!!! bat phead: ant cat n: • Given head and a pointer n • What expression gets node containing: • ant • bat • cat

DISCUSSION BREAK!!! bat phead: ant cat n: • Given head and a pointer n • What expression gets node containing: • ant => *head • bat => *(head->next), *n • cat => *(head->next->next), *(n->next)

Inserting Nodes tmp: jar phead: cat bat ant n:

Removing Nodes n: jar phead: cat bat ant

Using Nodes to make a List • LCLinkedList<T> • Member Variables • intnumItems; • Node<T>* phead; • Methods • same ADT as ArrayList • also has push_front, pop_front • same public methods

push_front phead: cat bat tmp: ant

push_front void push_front(const T& newItem){ Node* tmp = new Node(newItem); tmp->next = phead; phead = tmp; numItems++; }

push_back tmp: phead: cat bat ant

push_back void push_back(const T& newItem){ // list is empty (special case) if( phead == NULL ){ phead = new Node(newItem); }else{ // find the end of the list Node* pNode = phead; while(pNode->next != NULL){ pNode = pNode->next; } // add the item pNode->next = new Node(newItem); // increment the count numItems++; }}

Questions?

Recall: push_front void push_front(const T& newItem){ Node* tmp = new Node(newItem); tmp->next = phead; phead = tmp; numItems++; }

Recall: push_back void push_back(const T& newItem){ // list is empty (special case) if( phead == NULL ){ phead = new Node(newItem); }else{ // find the end of the list Node* pNode = phead; while(pNode->next != NULL){ pNode = pNode->next; } // add the item pNode->next = new Node(newItem); // increment the count numItems++; }}

Header Node phead: bat ant Header Eliminates special 1st node case for add and remove Create header at initialization Don’t count in size Ignore for contains

push_back (with header) void push_back(const T& newItem){ // find the end of the list Node* pNode = phead; while(pNode->next != NULL){ pNode = pNode->next; } // add the item pNode->next = new Node(newItem); // increment the count numItems++; }

DISCUSSION BREAK!!! • Can you modify LCLinkedList to push_back(…) in O(1)? • Hint: You can add member variables

Tail Pointer ptail: phead: bat ant Header push_back now constant time but, we must update tail pointer when adding to end

Tail Pointer ptail: phead: Header tail pointer points at header if list is empty

pop_back() ptail: phead: bat ant Header

pop_back() phead: ptail: bat ant Header

pop_back(with header and tail pointer) void pop_back(){ if( phead != ptail ){ // find the node just before ptail Node* pNode = phead; while(pNode->next != ptail){ pNode= pNode->next; } // remove the node pNode->next = NULL; ptail = pNode; // decrement the count numItems--; } }

pop_back() ptail: phead: bat ant Header Tail pointer close to the right position Still must iterate over nearly entire list Can we make remove at end O(1)?

DISCUSSION BREAK!!! • Can you modify LCLinkedList to pop_back() in O(1)? • Hint: You can modify the structure of the list

Doubly Linked List ptail: phead: ant bat Header • Node • Member variables • public: • T data; • Node<T> *next; • Node<T> *prev

Tail Node ptail: phead: Tail bat Header • Tail node • removes special cases just like header node • tail and header node called sentinels

pop_back() phead: ptail: Tail bat Header

pop_back(doubly linked with sentinels) void pop_back(){ if( phead->next != ptail ){ ptail->prev = ptail->prev->prev; ptail->prev->next = ptail; // decrement the count numItems--; } }

Linked Lists vs Array Lists • Space • linked needs two extra pointers per entry • linked needs two extra nodes • array may be up to N/2-1 too large • array may be way too big if some elements removed • array better small data items (e.g., ints) • linked way better for big items (e.g., classes) • too close to call for medium items (e.g., 3 ints)

Linked Lists vs Array Lists Time

Linked Lists vs Array Lists • Ease implementation • fairly close • linked list has more special cases and work-arounds for special cases

Questions?

Expanding List Functionality • What if we want to remove an item from the middle of a linked list? • remove at i • traverse list from beginning until we reach i • fix up the pointers • O(N)

Expanding List Functionality • What if we were already moving through the list? • e.g., remove all even numbers • if we did this internally (inside the list class),we would have a pointer to the node to remove • O(1)

Linked List Iterators • LCList::Iterator • Public nested class in LCList • Member data • Node<T> * currNode; • Friends • LCList<T>

Linked List Iterator Iterator::Iterator() : currNode(NULL){} Iterator::Iterator(Node* pNode) : currNode(pNode){} void Iterator::operator ++(){ currNode = currNode->next; } void Iterator::operator –-(){ currNode = currNode->prev; } T& Iterator::operator *(){ return currNode->data; }

Linked List Iterator Iterator LCList::begin(){ return Iterator(phead->next); } Iterator LCList::end(){ return Iterator(ptail); }

remove(Iterator) phead: Head bat cat Tail rat ant ptail: iter:

Linked List

Linked List

Presentation Transcript

Linked List

Linked List

Linked List

Linked List

Linked List

Linked List

Linked List

Linked List

Linked List

Linked List

Linked list

Linked List

Linked List

Linked List