Lecture 7 of Computer Science II

1. Lecture 7 ofComputer Science II Linked Lists Instructor: Mr.Ahmed Al Astal

2. LINKED LISTS Definition A sequence of nodes, each containing arbitrary data fields and one or two references ("links") pointing to the next and/or previous nodes.  Page 2

3. LINKED LISTS Definition • A linked list is a self-referential datatype because it contains a pointer or link to another datum of the same type. • Linked lists permit insertion and removal of nodes at any point in the list in constant time,[1] but do not allow random access. • Several different types of linked list exist: singly-linked lists, doubly-linked lists, and circularly-linked lists.  Page 3

4. tail head LINKED LISTS Applications • Navigating pages in a novel. • Presenting test questions. • Moving through a list of user settings on an application.  Page 4

5. tail head LINKED LISTS Vs. Arrays • Space usage: • Indexing • Insertion • Deletion  Page 5

6. tail head SINGLY LINKED LISTS Definition • A singly linked list is a concrete data structure consisting of a sequence of nodes • Each node stores • element • link to the next node  Page 6

7. SINGLY LINKED LISTS Implementing a Singly Linked List To implement a singly linked list, we define a Node class,, which specifies the type of objects stored at the nodes of the list. Given the Node class, we can define a class, SLinkedList,, defining the actual linked list. This class keeps a reference to the head node and a variable counting the total number of nodes.  Page 7

8. SINGLY LINKED LISTS Inserting at the Head 1. Allocate a new node 2. Insert new element 3. Have new node point to old head 4. Update head to point to new node  Page 8

9. SINGLY LINKED LISTS Removing at the Head 1. Update head to point to next node in the list 2. Allow garbage collector to reclaim the former first node  Page 9

10. SINGLY LINKED LISTS Inserting at the Tail 1. Allocate a new node 2. Insert new element 3. Have new node point to null 4. Have old last node point to new node 5. Update tail to point to new node  Page 10

11. SINGLY LINKED LISTS Removing at the Tail • Removing at the tail of a singly linked list is not efficient! • There is no constant-time way to update the tail to point to the previous node  Page 11

12. DOUBLY LINKED LISTS Doubly Linked Lists • There is a type of linked list that allows us to go in both directions—forward and reverse—in a linked list. It is the doubly linked list. • A node in a doubly linked list stores two references: • next link, which points to the next node in the list, • and a prev link, which points to the previous node in the list.  Page 12

13. DOUBLY LINKED LISTS Header and Trailer Sentinels • To simplify programming, it is convenient to add special nodes at both ends of a doubly linked list: • A header node just before the head of the list, • and a trailer node just after the tail of the list. • These "dummy" or sentinel nodes do not store any elements. The header has a valid next reference but a null prev reference, while the trailer has a valid prev reference but a null next reference.  Page 13

14. DOUBLY LINKED LISTS Removing the node at the end of a doubly linked list • Inserting or removing elements at either end of a doubly linked list is straight- forward to do. • Indeed, the prev links eliminate the need to traverse the list to get to the node just before the tail.  Page 14

15. DOUBLY LINKED LISTS Adding an element at the front • Likewise, we can easily perform an insertion of a new element at the beginning of a doubly linked list, as shown in the following Figure.  Page 15

16. DOUBLY LINKED LISTS Insertion in the Middle of a Doubly Linked List • Given a node v of a doubly linked list (which could be possibly the header but not the trailer), we can easily insert a new node z immediately after v. • Specifically, let w the be node following v. We execute the following steps: • make z's prev link refer to v • make z's next link refer to w • make w's prev link refer to z • make v's next link refer to z  Page 16

17. DOUBLY LINKED LISTS Removal in the Middle of a Doubly Linked List • We access the nodes u and w on either side of v using v's getPrev and getNext methods. • To remove node v, we simply have u and w point to each other instead of to v. • We refer to this operation as the linking out of v. • We also null out v's prev and next pointers so as not to retain old references into the list.  Page 17

18. Circularly Linked Lists Circularly Linked Lists • A circularly linked list has the same kind of nodes as a singly linked list. • But there is no head or tail in a circularly linked list. • For instead of having the last node's next pointer be null, in a circularly linked list, it points back to the first node. • Even though a circularly linked list has no beginning or end, we nevertheless need some node to be marked as a special node, which we call the cursor.  Page 18

19. Circularly Linked Lists Circularly Linked Lists • We can then define some simple update methods for a circularly linked list: • add(v): Insert a new node v immediately after the cursor; if the list is empty, then v becomes the cursor and its next pointer points to itself. • remove(): Remove and return the node v immediately after the cursor (not the cursor itself, unless it is the only node); if the list becomes empty, the cursor is set to null. • advance(): Advance the cursor to the next node in the list. • Look at the Java implementation of a circularly linked list Page No. 153 (Text Book)  Page 19

20. Circularly Linked Lists Sorting a Linked List Following is the pseudo code for the insertion-sort algorithm for a doubly linked list.  Page 20