CC 215 DATA STRUCTURES LINKED LISTS Dr. Manal Helal - Fall 2014 Lecture 3 AASTMT Engineering and Technology College 1

Readings 2  Reading  Section 3.1 ADT (recall, lecture 1): Abstract Data Type (ADT): Mathematical description of an object with set of operations on the object.  Section 3.2 The List ADT

List ADT 3  What is a List?  Ordered sequence of elements A 1, A 2, …, A N  Elements may be of arbitrary type, but all are of the same type  Common List operations are:  Insert, Find, Delete, IsEmpty, IsLast, FindPrevious, First, Kth, Last, Print, etc.

Simple Examples of List Use 4  Polynomials  x x 85  Unbounded Integers   Text  “ This is an example of text ”

List Implementations 5  Two types of implementation:  Array-Based  Pointer-Based

List: Array Implementation 6  Basic Idea:  Pre-allocate a big array of size MAX_SIZE  Keep track of current size using a variable count  Shift elements when you have to insert or delete ANAN …A4A4 A3A3 A2A2 A1A1 MAX_SIZE-1count-1…3210

List: Array Implementation 7 FEDCBA MAX_SIZE Insert Z in kth position EDCZBA MAX_SIZE F 6

Array List Insert Running Time 8  Running time for N elements?  On average, must move half the elements to make room – assuming insertions at positions are equally likely  Worst case is insert at position 0. Must move all N items one position before the insert  This is O(N) running time. Probably too slow

Review Big Oh Notation 9  T(N) = O(f(N)) if there are positive constants c and n 0 such that: T(N) n 0  T(N) = O(N) linear

List: Pointer Implementation 10  Basic Idea:  Allocate little blocks of memory (nodes) as elements are added to the list  Keep track of list by linking the nodes together  Change links when you want to insert or delete Value NULL L node ValueNext node Next

© 2005 Pearson Addison- Wesley. All rights reserved 4-11 Preliminaries Figure 4.1 a) A linked list of integers; b) insertion; c) deletion

© 2005 Pearson Addison- Wesley. All rights reserved 4-12 Pointer-Based Linked Lists  A node in a linked list is usually a struct struct Node{ int item; Node *next; };  A node is dynamically allocated Node *p; p = new Node; Figure 4.6 A node

© 2005 Pearson Addison- Wesley. All rights reserved 4-13 Pointer-Based Linked Lists  The head pointer points to the first node in a linked list  If head is NULL, the linked list is empty

© 2005 Pearson Addison- Wesley. All rights reserved 4-14 Pointer-Based Linked Lists Figure 4.7 A head pointer to a list Figure 4.8 A lost cell

© 2005 Pearson Addison- Wesley. All rights reserved 4-15 Displaying the Contents of a Linked List  Reference a node member with the -> operator p->item;  A traverse operation visits each node in the linked list  A pointer variable cur keeps track of the current node for (Node *cur = head; cur != NULL; cur = cur->next) cout item << endl;

© 2005 Pearson Addison- Wesley. All rights reserved 4-16 Displaying the Contents of a Linked List Figure 4.9 The effect of the assignment cur = cur->next

© 2005 Pearson Addison- Wesley. All rights reserved 4-17 Deleting a Specified Node from a Linked List  Deleting an interior node prev->next=cur->next;  Deleting the first node head=head->next;  Return deleted node to system cur->next = NULL; delete cur; cur=NULL;

© 2005 Pearson Addison- Wesley. All rights reserved 4-18 Deleting a Specified Node from a Linked List Figure 4.10 Deleting a node from a linked list Figure 4.11 Deleting the first node

© 2005 Pearson Addison-Wesley. All rights reserved 4-19 Inserting a Node into a Specified Position of a Linked List  To insert a node between two nodes newPtr->next = cur; prev->next = newPtr; Figure 4.12 Inserting a new node into a linked list

© 2005 Pearson Addison-Wesley. All rights reserved 4-20 Inserting a Node into a Specified Position of a Linked List  To insert a node at the beginning of a linked list newPtr->next = head; head = newPtr; Figure 4.13 Inserting at the beginning of a linked list

© 2005 Pearson Addison-Wesley. All rights reserved 4-21 Inserting a Node into a Specified Position of a Linked List  Inserting at the end of a linked list is not a special case if cur is NULL newPtr->next = cur; prev->next = newPtr; Figure 4.14 Inserting at the end of a linked list

© 2005 Pearson Addison- Wesley. All rights reserved 4-22 Inserting a Node into a Specified Position of a Linked List  Determining the point of insertion or deletion for a sorted linked list of objects for(prev = NULL, cur= head; (cur != null)&& (newValue > cur->item); prev = cur, cur = cur->next);

© 2005 Pearson Addison- Wesley. All rights reserved 4-23 A Pointer-Based Implementation of the ADT List  Public methods  isEmpty  getLength  insert  remove  retrieve  Private method  find  Private Data Members  head  size  Local variables to member functions  cur  prev

© 2005 Pearson Addison- Wesley. All rights reserved 4-24 Constructors and Destructors  Default constructor initializes size and head  Copy constructor allows a deep copy  Copies the array of list items and the number of items  A destructor is required for dynamically allocated memory

© 2005 Pearson Addison- Wesley. All rights reserved 4-25 Comparing Array-Based and Pointer-Based Implementations  Size  Increasing the size of a resizable array can waste storage and time  Storage requirements  Array-based implementations require less memory than a pointer-based ones

© 2005 Pearson Addison- Wesley. All rights reserved 4-26 Comparing Array-Based and Pointer-Based Implementations  Access time  Array-based: constant access time  Pointer-based: the time to access the i th node depends on i  Insertion and deletions  Array-based: require shifting of data  Pointer-based: require a list traversal

© 2005 Pearson Addison- Wesley. All rights reserved 4-27 Saving and Restoring a Linked List by Using a File  Use an external file to preserve the list between runs  Do not write pointers to a file, only data  Recreate the list from the file by placing each item at the end of the list  Use a tail pointer to facilitate adding nodes to the end of the list  Treat the first insertion as a special case by setting the tail to head

Pointer Implementation Issues 28  Whenever you break a list, your code should fix the list up as soon as possible  Draw pictures of the list to visualize what needs to be done  Pay special attention to boundary conditions:  Empty list  Single item – same item is both first and last  Two items – first, last, but no middle items  Three or more items – first, last, and middle items

Pointer List Insert Running Time 29  Running time for N elements?  Insert takes constant time (O(1))  Does not depend on input size  Compare to array based list which is O(N)

© 2005 Pearson Addison-Wesley. All rights reserved 4-30 Circular Linked Lists  Last node references the first node  Every node has a successor  No node in a circular linked list contains NULL Figure 4.25 A circular linked list

© 2005 Pearson Addison-Wesley. All rights reserved 4-31 Circular Linked Lists Figure 4.26 A circular linked list with an external pointer to the last node

© 2005 Pearson Addison- Wesley. All rights reserved 4-32 Doubly Linked Lists  Each node points to both its predecessor and its successor  Circular doubly linked list  precede pointer of the dummy head node points to the last node  next reference of the last node points to the dummy head node  No special cases for insertions and deletions

© 2005 Pearson Addison- Wesley. All rights reserved 4-33 Doubly Linked Lists Figure 4.28 A doubly linked list

© 2005 Pearson Addison- Wesley. All rights reserved 4-34 Doubly Linked Lists Figure 4.29 (a) A circular doubly linked list with a dummy head node (b) An empty list with a dummy head node

© 2005 Pearson Addison- Wesley. All rights reserved 4-35 Doubly Linked Lists  To delete the node to which cur points (cur->precede)->next = cur->next; (cur->next)->precede = cur->precede;  To insert a new node pointed to by newPtr before the node pointed to by cur newPtr->next = cur; newPtr->precede = cur->precede; cur->precede = newPtr; newPtr->precede->next = newPtr;

© 2005 Pearson Addison- Wesley. All rights reserved 4-36 Summary  Each pointer in a linked list is a pointer to the next node in the list  Algorithms for insertions and deletions in a linked list involve traversing the list and performing pointer changes  Inserting a node at the beginning of a list and deleting the first node of a list are special cases

© 2005 Pearson Addison- Wesley. All rights reserved 4-37 Summary  Recursion can be used to perform operations on a linked list  In a circular linked list, the last node points to the first node  Dummy head nodes eliminate the special cases for insertion into and deletion from the beginning of a linked list

Assignment 1  Add to the LList class implementation a member function to reverse the order of the elements on the list. Your algorithm should run in Θ (n) time for a list of n elements. 38