Data Structures and Algorithms

Transcription

1 Data Structures and Algorithms First Semester 2017/2018 Linked Lists Eng. Anis Nazer

2 Linked List ADT Is a list of nodes Each node has: data (can be any thing, int, char, Person, Point, day,...) link to the next node in the list (usually a pointer) Operations on a Linked list insert a node into the list delete a node from the list search the list for a node sort the list

3 Linked List Node: List: head data data data data NULL

4 Implementation We can use C++ classes to implement a linked list Node class: class Node { public: int data; Node *link; Node(int d = 0, Node *p = NULL); ; Node::Node(int d, Node * p ) { data = d; link = p;

5 Example 1 What does the following code do? Node x(15); variable x 15 NULL

6 Example 1 What does the following code do? Node x(15); Node y; variable x 15 NULL variable y 0 NULL

7 Example 1 What does the following code do? Node x(15); Node y; Node *h = &x; h variable x 15 NULL variable y 0 NULL

8 Example 1 What does the following code do? Node x(15); Node y; Node *h = &x; h >link = &y; h variable x 15 NULL variable y 0 NULL

9 Example 1 What does the following code do? Node x(15); Node y; Node *h = &x; h >link = &y; cout << h >data << endl; cout << h >link >data << endl; h variable x 15 variable y 0 NULL

10 Example 1 What does the following code do? Node x(15); Node y; Node *h = &x; h >link = &y; cout << h >data << endl; cout << h >link >data << endl; output window 15 0

11 Example 2 What does the following code do? Node *head, *p; head = new Node(10); cout << head >data << endl; p = new Node(7); head >link = p; cout << head >data << endl; cout << head >link >data << endl; //cout << head >link >link >data << endl; // problem why? p = new Node(9); head >link >link = p; cout << head >link >link >data << endl;

12 Example 2 Node *head, *p; head p head = new Node(10); head p 10 NULL cout << head >data << endl; output window 10

13 Example 2 p = new Node(7); head p 10 NULL 7 NULL head >link = p; head 10 p 7 NULL cout << head >data << endl; cout << head >link >data << endl; output window

14 Example 2 p = new Node(9); head p 10 7 NULL 9 NULL head >link >link = p; head 10 7 p 9 NULL cout << head >link >link >data << endl; output window

15 Exercises Write statements to read 10 numbers from the user and store them in a linked list Write a loop to display the elements of a linked list. Assume the pointer head points to the first node in the list Write a loop to display the number of elements of a linked list. Assume the pointer head points to the first node in the list

16 Example 3 p = head; head p NULL head = head >link; p head NULL delete p; p head 7 9 NULL

17 Example head p 7 9 NULL p = head; head = head >link; delete p; p head 7 9 NULL p head 9 NULL

18 Example head p 9 NULL p = head; head = head >link; delete p; p head 9 NULL p head NULL

19 Example 2 how to delete the list? // delete objects p = head; head = head >link; delete p; p = head; head = head >link; delete p; p = head; head = head >link; delete p; // delete first node // delete second node // delete third node

20 Example 3 What does the following code do? Node *head; head = new Node( 3); head = new Node(5, head); head = new Node(11, head); head = new Node(19, head); Node *p; p = head; while ( p!= NULL ) { cout << p >data << endl; p = p >link; while ( head!= NULL ) { p = head; head = head >link; delete p;

21 Linked List Class UML head : Node * tail : Node * LinkedList + LinkedList() + ~Linked() + isempty() const : bool + addtohead(int) : void + addtotail(int) : void + deletefromhead() : int + deletefromtail() : int + deletenode(int) : void + isinlist(int) const : bool + display() const : void

22 LinkedList class class LinkedList { public: LinkedList(); ~LinkedList(); bool isempty() const; void addtohead(int d); void addtotail(int d); int deletefromhead(); int deletefromtail(); void deletenode(int d); bool isinlist(int) const; void display() const; private: Node *head, *tail; ;

23 Example Write statements to: create a linked list called L1 add the following to the list: 3, 9, 7, 8, 5 display the data in the last node and delete it delete the node containing data 7 display the contentes of the list

24 LinkedList class constructor and destructor LinkedList::LinkedList( ) { head = tail = NULL; LinkedList::~LinkedList( ) { Node * p; while ( head!= NULL ) { p = head; head = head >link; delete p;

25 LinkedList class isempty(), display() : bool LinkedList::isEmpty() const { return (head == NULL); void LinkedList::display() const { Node * p = head; while ( p!= NULL ) { cout << p >data << endl; p = p >link;

26 LinkedList class isinlist() : bool LinkedList::isInList(int d) const { Node * p = head; while ( p!= NULL ) { if ( p >data == d ) return true; p = p >link; return false;

27 LinkedList class addtohead() 1) new node 2) link of new node equals head 3) update head to new node 4) special case: if list is empty, tail should be updated to new node void LinkedList::addToHead(int d) { head = new Node(d, head); if ( tail == NULL ) tail = head;

28 LinkedList class addtotail() : new node if list is empty head, tail should be updated to new node if list is not empty: update tail link to point to new node update tail void LinkedList::addToTail(int d) { Node *p = new Node(d); if ( isempty() ) { head = tail = p; else { tail >link = p; tail = p;

29 LinkedList class deletefromhead() : get data p points to head if a single node: set head and tail to NULL otherwise: update head to next node delete p, and return data int LinkedList::deleteFromHead() { int d = head >data; Node *p = head; if ( head == tail ) { head = tail = NULL; else { head = head >link; delete p; return d;

30 LinkedList class deletefromtail(): Two cases: if a single node: delete node and update head, tail, and return data otherwise we need a pointer to the previous node Steps: start with head and keep moving in the list until you reach tail delete node, update tail, return data

31 LinkedList class deletefromtail(): int LinkedList::deleteFromTail() { int d = tail >data; if ( head == tail ) { delete head; head = tail = NULL; else { Node *p = head; while ( p >link!= tail ) p = p >link; delete tail; tail = p; tail >link = NULL; return d;

32 LinkedList class deletenode(): if list is empty, do nothing if list is not empty, the following cases should be considered: one node more than one node where target node is: first node middle node last node

33 LinkedList class deletenode(): if (! isempty() ) { if ( head == tail && head >data == d ) { // case 1 else { if ( head >data == d ) { // case 2 else { // case 3 // if target is tail, update tail

34 LinkedList class deletenode(): list is not empty, and we have a single node (case 1): if (! isempty() ) { if ( head == tail && head >data == d ) { delete head; head = tail = NULL; else { //...

35 LinkedList class deletenode(): list is not empty, and we have more than one node, head is the target node (case 2): if (! isempty() ) { if ( head == tail && head >data == d ) { //... else { if ( head >data == d ) { Node *p = head; head = head >link; delete p; else { //...

36 LinkedList class deletenode(): list is not empty, and we have more than one node, target is not head node (case 3): we need two pointers, p : to point to a node q : to point to the previous node, (predecessor) Steps: traverse the list until you reach the target make q->link point to the node after p if the deleted node is the tail, update the tail

37 LinkedList class deletenode(): list is not empty, and we have more than one node, target is not head node (case 3): else { // case 3 Node *p, *q; // q predecessor to p q = head; p = head >link; while(p!= NULL && p >data!= d) { p = p >link; q = q >link; if ( p!= NULL ) { q >link = p >link; if ( p == tail ) tail = q; delete p;

38 LinkedList class void LinkedList::deleteNode(int d) { if (! isempty() ) { if ( head == tail && head >data == d ) { // case 1 delete head; head = tail = NULL; else { if ( head >data == d ) { // case 2 Node *p = head; head = head >link; delete p; else { // case 3 Node *p, *q; // q predecessor to p q = head; p = head >link; while(p!= NULL && p >data!= d) { p = p >link; q = q >link; if ( p!= NULL ) { q >link = p >link; if ( p == tail ) tail = q; delete p;

39 Example What does the following code do? int main() { LinkedList S; for (int i=0 ; i < 8 ; i++) S.addToHead(i); S.deleteNode(2); S.deleteNode(7); for (int i=0; i < 4 ; i++) cout << S.deleteFromTail() << endl; cout << endl; S.display(); return 0;

40 Linked List Complexities What is the complexity of the following operations for a linked list? addtohead() addtotail() deletefromhead() deletefromtail() deletenode() isinlist() display()

41 Advantages What are the advantages of using a linked list? Why not just use an array?