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Data Structures and Algorithms. (Linked List) Instructor: Quratulain . 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.

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Data structures and algorithms

Data Structures and Algorithms

(Linked List)

Instructor: Quratulain


Why need link list
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.

CSE 246 Data Structures and Algorithms Quratulain


Introduction to link list
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

node

elem

CSE 246 Data Structures and Algorithms Quratulain


Anatomy of a linked list

myList

a

b

c

d

Anatomy of a linked list

  • A linked list consists of:

    • A sequence of nodes

  • 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

CSE 246 Data Structures and Algorithms Quratulain


More terminology
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 Quratulain


Pointers and references
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 intcan only refer to an array of int

  • CSE 246 Data Structures and Algorithms Quratulain


    List implementation using linked lists
    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.

    CSE 246 Data Structures and Algorithms Quratulain


    Self referential class

    3

    2

    100

    500

    Data member and pointer

    NULL pointer (points to nothing)

    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

    class node { int data; node next;…}

    CSE 246 Data Structures and Algorithms Quratulain


    Creating references
    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 Quratulain


    Creating links in java

    myList:

    44

    97

    23

    17

    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);

    CSE 246 Data Structures and Algorithms Quratulain


    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)

    myList

    a

    b

    c

    d

    Singly-linked lists

    CSE 246 Data Structures and Algorithms Quratulain


    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

    Singly-linked lists in Java

    CSE 246 Data Structures and Algorithms Quratulain


    Singly linked list nodes in java
    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 Quratulain


    Creating a simple list

    numerals

    two

    three

    one

    Creating a simple list

    • To create the list ("one", "two", "three"):

    • SLinkListnumerals = new SLinkList();

    • Numerals.start= new node("one", new node("two", new node("three", null)));

    CSE 246 Data Structures and Algorithms Quratulain


    Traversing a slinklist
    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 SLinkListclass

    CSE 246 Data Structures and Algorithms Quratulain


    Traversing a slinklist animation

    curr

    numerals

    two

    three

    one

    Traversing a SLinkList(animation)

    CSE 246 Data Structures and Algorithms Quratulain


    Inserting a node into a slinklist
    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 Quratulain


    Inserting as a new first element
    Inserting as a new first element

    • This is probably the easiest method to implement

    • In class SLinkList(not node):

    • 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 Quratulain


    Inserting a node after a given value
    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 Quratulain


    Inserting after animation

    n

    2.5

    numerals

    two

    three

    one

    Inserting after (animation)

    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 Quratulain


    Deleting a node from a slinklist
    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 Quratulain


    Deleting an element from a sll

    numerals

    two

    three

    one

    numerals

    two

    three

    one

    Deleting an element from a SLL

    • 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 Quratulain


    Deleting from a slinklist
    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 Quratulain


    Garbage collection
    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 Quratulain


    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

    a

    b

    c

    Doubly-linked lists

    CSE 246 Data Structures and Algorithms Quratulain


    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)

    DLinkListscompared to SLinkLists

    CSE 246 Data Structures and Algorithms Quratulain


    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...

    }

    Constructing SLinkListsand DLinkLists

    CSE 246 Data Structures and Algorithms Quratulain


    Dlinklist nodes in java
    DLinkListnodes 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 Quratulain


    Deleting a node from a dlinklist

    Node deletion from a DLinkListinvolves changing two links

    Deletion of the first node or the last node is a special case

    Garbage collection will take care of deleted nodes

    myDLinkList

    a

    b

    c

    Deleting a node from a DLinkList

    CSE 246 Data Structures and Algorithms Quratulain


    Other operations on linked lists
    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 nth element—you have to count your way through a lot of other elements

    CSE 246 Data Structures and Algorithms Quratulain


    Analyzing the singly linked list
    Analyzing the Singly Linked List

    Method

    Time

    Inserting at the head

    O(1)

    Inserting at the tail

    O(1)

    Deleting at the head

    O(1)

    Deleting at the tail

    O(n)

    CSE 246 Data Structures and Algorithms Quratulain


    Types of linked lists
    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

    CSE 246 Data Structures and Algorithms Quratulain


    Circular link list

    10

    20

    40

    70

    55

    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

    Rear

    CSE 246 Data Structures and Algorithms Quratulain


    Circular linked lists
    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 Quratulain


    Issues with circular list
    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 Quratulain


    The josephus problem
    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 Quratulain


    Solution
    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 Quratulain


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