Data Structures and Algorithms Lecture 7,8 and 9 (Linked List) Instructor: Quratulain Date: 25, 29 September and 2 October, 2009 Faculty of Computer Science, IBA

Why need Link List? The array implementation has serious drawbacks: ◦ you must know the maximum number of items in your collection when you create it. ◦ must be in continuous manner. We can use a structure called a linked list to overcome this limitation. 2 CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Introduction to Link List The linked list is a very flexible dynamic data structure A programmer need not worry about how many items a program will have to accommodate In a linked list, each item is allocated space as it is added to the list. A link is kept with each item to the next item in the list. next elem node 3 CSE 246 Data Structures and Algorithms Fall2009 Quratulain

4 Anatomy of a linked list A linked list consists of: ◦ A sequence of nodes abcd Each node contains a value and a link (pointer or reference) to some other node The last node contains a null link The list may have a header myList CSE 246 Data Structures and Algorithms Fall2009 Quratulain

5 More terminology A node’s successor is the next node in the sequence ◦ The last node has no successor A node’s predecessor is the previous node in the sequence ◦ The first node has no predecessor A list’s length is the number of elements in it ◦ A list may be empty (contain no elements) CSE 246 Data Structures and Algorithms Fall2009 Quratulain

6 Pointers and references In C and C++ we have “pointers,” while in Java we have “references” These are essentially the same thing The difference is that C and C++ allow you to modify pointers in arbitrary ways, and to point to anything In Java, a reference is more of a “black box,” or ADT Available operations are:  dereference (“follow”)  copy  compare for equality There are constraints on what kind of thing is referenced: for example, a reference to an array of int can only refer to an array of int CSE 246 Data Structures and Algorithms Fall2009 Quratulain

List Implementation using Linked Lists Linked list ◦ Linear collection of self-referential class objects, called nodes ◦ Connected by pointer links ◦ Accessed via a pointer to the first node of the list ◦ Link pointer in the last node is set to null to mark the list’s end Use a linked list instead of an array when ◦ You have an unpredictable number of data elements ◦ You want to insert and delete quickly. 7 CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Self-Referential Class Self-referential structures ◦ Class that contains a pointer to a class of the same type ◦ Can be linked together to form useful data structures such as lists, queues, stacks and trees ◦ Terminated with a NULL reference Diagram of two self-referential class objects linked together NULL pointer (points to nothing)Data member and pointer 500 … 32 class node { int data; node next; …} CSE 246 Data Structures and Algorithms Fall2009 Quratulain

9 Creating references The keyword new creates a new object, but also returns a reference to that object For example, Person p = new Person("John") ◦ new Person("John") creates the object and returns a reference to it ◦ We can assign this reference to p, or use it in other ways CSE 246 Data Structures and Algorithms Fall2009 Quratulain

10 Creating links in Java class node { int value; node next; node (int v, node n) { // constructor value = v; next = n; } } node temp = new node(17, null); temp = new node(23, temp); temp = new node(97, temp); node myList = new node(44, temp); myList: CSE 246 Data Structures and Algorithms Fall2009 Quratulain

11 Singly-linked lists Here is a singly-linked list: Each node contains a value and a link to its successor (the last node has no successor) The header points to the first node in the list (or contains the null link if the list is empty) abcd myList CSE 246 Data Structures and Algorithms Fall2009 Quratulain

12 Singly-linked lists in Java public class SLinkList { Private node start; public SLinkList() { this.first = null; } // methods... Public void Addnode(int d){ … } Public void getlistnode(){ …} } This class actually describes the header of a singly-linked list However, the entire list is accessible from this header Users can think of the SLL as being the list ◦ Users shouldn’t have to worry about the actual implementation CSE 246 Data Structures and Algorithms Fall2009 Quratulain

13 Singly Linked List nodes in Java public class node { protected Object element; protected node next; protected node(Object elem, node next) { this.element = elem; this.next = next; } } CSE 246 Data Structures and Algorithms Fall2009 Quratulain

14 Creating a simple list To create the list ("one", "two", "three"): SLinkList numerals = new SLinkList(); Numerals.start = new node("one", new node("two", new node("three", null))); threetwoone numerals CSE 246 Data Structures and Algorithms Fall2009 Quratulain

15 Traversing a SLinkList The following method traverses a list (and prints its elements): public void printFirstToLast() { for (node curr = start; curr != null; curr = curr.next) { System.out.print(curr.element + " "); } } You would write this as an instance method of the SLinkList class CSE 246 Data Structures and Algorithms Fall2009 Quratulain

16 Traversing a SLinkList (animation) threetwo one numerals curr CSE 246 Data Structures and Algorithms Fall2009 Quratulain

17 Inserting a node into a SLinkList There are many ways you might want to insert a new node into a list: ◦ As the new first element ◦ As the new last element ◦ Before a given node (specified by a reference) ◦ After a given node ◦ Before a given value ◦ After a given value All are possible, but differ in difficulty CSE 246 Data Structures and Algorithms Fall2009 Quratulain

18 Inserting as a new first element This is probably the easiest method to implement In class SLinkList (not n ode ): void insertAtFront(node n) { n.next = this.start; this.start = n; } Notice that this method works correctly when inserting into a previously empty list CSE 246 Data Structures and Algorithms Fall2009 Quratulain

19 Inserting a node after a given value void insertAfter(Object obj, node n) { for(node here =this.start; here != null;here = here.next) { if (here.element.equals(obj)) { n.next = here. next; here. next = n; return; } // if } // for // Couldn't insert--do something reasonable! } CSE 246 Data Structures and Algorithms Fall2009 Quratulain

20 Inserting after (animation) threetwo one numerals 2.5 n Find the node you want to insert after First, copy the link from the node that's already in the list Then, change the link in the node that's already in the list CSE 246 Data Structures and Algorithms Fall2009 Quratulain

21 Deleting a node from a SLinkList In order to delete a node from a SLL, you have to change the link in its predecessor This is slightly tricky, because you can’t follow a pointer backwards Deleting the first node in a list is a special case, because the node’s predecessor is the list header CSE 246 Data Structures and Algorithms Fall2009 Quratulain

22 Deleting an element from a SLL threetwo one numerals threetwo one numerals To delete the first element, change the link in the header To delete some other element, change the link in its predecessor Deleted nodes will eventually be garbage collected CSE 246 Data Structures and Algorithms Fall2009 Quratulain

23 Deleting from a SLinkList public void delete(node del) { node temp = del. next; // If del is first node, change link in header if (del == first) first = temp; else { // find predecessor and change its link node pred = first; while (pred.next != del) pred = pred.next; pred. next = temp; } CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Garbage Collection What happens to objects that have no references to them? They are inaccessible to the program Java system will remove them and recycle the memory (usually when low on memory) How this is done is up to the implementation I’ll describe two approaches: ◦ Reference counting (has problems with cycles) ◦ Mark and sweep, or tracing Compaction is also important CSE 246 Data Structures and Algorithms Fall2009 Quratulain24

25 Doubly-linked lists Here is a doubly-linked list (DLinkList): Each node contains a value, a link to its successor (if any), and a link to its predecessor (if any) The header points to the first node in the list and to the last node in the list (or contains null links if the list is empty) myDLinkList ab c CSE 246 Data Structures and Algorithms Fall2009 Quratulain

26 DLinkLists compared to SLinkLists Advantages: ◦ Can be traversed in either direction (may be essential for some programs) ◦ Some operations, such as deletion and inserting before a node, become easier Disadvantages: ◦ Requires more space ◦ List manipulations are slower (because more links must be changed) ◦ Greater chance of having bugs (because more links must be manipulated) CSE 246 Data Structures and Algorithms Fall2009 Quratulain

27 Constructing SLinkLists and DLinkLists public class SLinkList { private node first; public SLinkList () { this.first = null; } // methods... } public class DLinkList { private node first; private node last; public DLinkList () { this.first = null; this.last = null; } // methods... } CSE 246 Data Structures and Algorithms Fall2009 Quratulain

28 DLinkList nodes in Java public class node { protected Object element; protected node pred, succ; protected node(Object elem, node pred, node succ) { this.element = elem; this.pred = pred; this.succ = succ; } } CSE 246 Data Structures and Algorithms Fall2009 Quratulain

29 Deleting a node from a DLinkList Node deletion from a DLinkList involves changing two links Deletion of the first node or the last node is a special case Garbage collection will take care of deleted nodes myD LinkList ab c CSE 246 Data Structures and Algorithms Fall2009 Quratulain

30 Other operations on linked lists Most “algorithms” on linked lists—such as insertion, deletion, and searching—are pretty obvious; you just need to be careful Sorting a linked list is just messy, since you can’t directly access the n th element—you have to count your way through a lot of other elements CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Analyzing the Singly Linked List MethodTime Inserting at the head Inserting at the tail Deleting at the head Deleting at the tail O(1) O(n) 31 CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Types of linked lists Types of linked lists: ◦ Singly linked list  Begins with a pointer to the first node  Terminates with a null pointer  Only traversed in one direction ◦ Circular, singly linked  Pointer in the last node points back to the first node ◦ Doubly linked list  Two “start pointers” – first element and last element  Each node has a forward pointer and a backward pointer  Allows traversals both forwards and backwards ◦ Circular, doubly linked list  Forward pointer of the last node points to the first node and backward pointer of the first node points to the last node 32 CSE 246 Data Structures and Algorithms Fall2009 Quratulain

Circular Link List A Circular Linked List is a special type of Linked List It supports traversing from the end of the list to the beginning by making the last node point back to the head of the list A Rear pointer is often used instead of a Head pointer CSE 246 Data Structures and Algorithms Fall2009 Quratulain33 Rear

Circular linked lists Circular linked lists are usually sorted Circular linked lists are useful for playing video and sound files in “looping” mode They are also a stepping stone to implementing graphs. CSE 246 Data Structures and Algorithms Fall2009 Quratulain34

Issues with circular list How do you know when you’re done? ◦ Make sure you save the head reference. ◦ When (cur.next == head) you’ve reached the end How are insertion and deletion handled? ◦ No special cases! ◦ Predecessor to head node is the last node in the list. CSE 246 Data Structures and Algorithms Fall2009 Quratulain35

The Josephus Problem (One of many variations…) The founder of a startup is forced to lay off all but one employee. Not having any better way to decide, he arranges all employees in a circle and has them count off. The 10th employee in the circle is laid off, and the count begins again. The last person is not laid off. If there are N employees, where should you sit to avoid being laid off? CSE 246 Data Structures and Algorithms Fall2009 Quratulain36

Solution Model the circle of employees as a circular linked list Implement the counting off process, and delete the 10th employee each time After N-1 people are deleted, there should be only one employee left. That employee’s original position number is the solution to the problem. CSE 246 Data Structures and Algorithms Fall2009 Quratulain37