1. Linked Lists

2. 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 is accessed via a header (which contains a reference to the first node in the list) myList

3. 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)

4. Singly-linked lists Here is a singly-linked list (SLL): 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

5. Building a linked list The textbook by Lafore defines two classes: –Link, which holds a data value and a pointer I would have called this class “ Node ” or maybe “ Cell ” –LinkList, which is the header for a list, and just holds a reference ( first ) to the first Link in the list Lafore is avoiding the better name LinkedList, in order to avoid conflicts with the class java.util.LinkedList The user refers to LinkList and (if the ADT is done properly) neither knows nor cares about the Link class Lafore does a poor job of hiding the Link class from the user, probably for clarity (better information hiding requires some slightly tricky Java programming)

6. The LinkList class The most important thing in the LinkList class is a reference to the first node in the list: –public class LinkList { private Link first; The class has a constructor: –LinkList() { first = null; } And some methods: –public boolean isEmpty() { return first == null; } –public void insertFirst(int data) {...} –public Link deleteFirst() {...} –public Link find(int key) {...} –public Link delete(int key) {...} –...and several others

7. About headers Since the header of a list is what the user sees, it is usually given a name like LinkedList (as if it were the entire thing), while the actual nodes in the list are given less impressive-sounding names (like Link, or Node ) A list header always contains a reference to the first element of the list –If this is all it contains, we don’t really need it; just keep a reference to the first element of the list In this case, the list nodes would have a name like LinkedList, so that our reference can be of type LinkedList Typically, a list header contains other information as well, such as how many nodes are in the list, or a reference to the last node in the list

8. Creating a simple list To create the list [1, 2, 3]: 321 numbers LinkList numbers = new LinkList(); numbers.insertFirst(3); numbers.insertFirst(2); numbers.insertFirst(1);

9. Traversing a SLL The following method traverses a list (and prints its elements): public void print() { for (Link curr = first; curr != null; curr = curr.next) { System.out.print(next.element + " "); } } This would be an instance method of the LinkList class

10. Traversing a SLL (animation) 32 1 numbers curr

11. Inserting a node into a SLL 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

12. Inserting as a new first element This is probably the easiest method to implement public void insertFirst(int data) { Link newLink = new Link(data); newLink.next = first; first = newLink; } Notice that this method works correctly when inserting into a previously empty list

13. Inserting a node after a given value public void insertAfter(int oldData, int newData) { for (Link current = first; current != null; current = current.next) { if (current.data == oldData) { Link newLink = new Link(newData); newLink.next = current.next; current.next = newLink; return; } } System.out.print("Not found"); }

14. Inserting after (animation) 42 1 numbers 3 node 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

15. Deleting a node from a SLL 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

16. Deleting an element from a SLL 32 1 numbers 32 1 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

17. Deleting from a SLL public void delete(int badData) { if (first == null) return; // not in list (list is empty) if (first.data == badData) { first = first.next; return; } for (Link current = first; current.next != null; current = current.next) { if (current.next.data == badData) { current.next = current.next.next; return; } } // not in list }

18. Doubly-linked lists Here is a doubly-linked list (DLL): 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) myDLL ab c

19. DLLs compared to SLLs 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)

20. Deleting a node from a DLL Node deletion from a DLL 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 myDLL ab c

21. 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

22. The End