Prelude...

  • Data structures created with references are very flexible !

    Consequently:

    • There are many varieties of linked lists....

  • We will first study the singly linked linked list where each node has a single reference (link):

  • How to approach the material on list manipulation:

    • Do not memorize (too many varieties of lists !)

    • Understand what an operation must achieve

    • Devise the steps to manipulate the links to achieve that outcome

Operations on a Linked List

  • The linked list is a data structure used to store information

  • To be useful as a data structure, the linked list must support the following operations:

    • Create an empty linked list

    • Insert a data item into the linked list

      • Insert it at the beginning of the linked list
      • Insert it at the end of the linked list

    • Delete a data item from the linked list

      • Insert the element at the beginning of the linked list
      • Insert the element at the end of the linked list

    • Searching for some data item in the linked list

      • Search by its index
      • Seach by its key value

Introduction to linked list mainipulation

  • I will use the following simple Node class as demo: 

     public class Node
 {
    String item;
    Node   next;
 }

  • I will first create a linked list in an unconventional way:

    • I will chain a number of Node objects explicitly using helper variables

      (In the actual list manipulation operations, we do not use explicit helper variables)

  • Then, I want to show you the most fundamental algorithm on linked list, which is:

    • The linked list traversal algorithm

    Many list manipulation algorithms will use the list traversal algorithm

Constructing a linked list using explicit helper reference variables

  • Let's create the following linked list: 

    


   I will write staements here that achieves
   the effect in the above diagram

   Key: do not memorize the statements

        understand why the statements will
        achieve a particular effect

Constructing a linked list using explicit helper reference variables

  • Let's start with a clean slate: 

    public class Demo
{
   public static Node first; // Define the first variable

           // first references to a Node object
           // Therefore, its data type is Node






}

Constructing a linked list using explicit helper reference variables

  • Define some helper variables to hold Node objects: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       Node help1;
       Node help2;
       Node help3;



}

Constructing a linked list using explicit helper reference variables

  • Create/allocate the Node objects: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       Node help1 = new Node();
       Node help2 = new Node();
       Node help3 = new Node();



}

Constructing a linked list using explicit helper reference variables

  • Store the proper string in each Node: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       ...
       help1.item = "to";
       help2.item = "be";
       help3.item = "or";


}

Constructing a linked list using explicit helper reference variables

  • Use the next variable in each Node to create this chain: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       ...
       help1.next = help2;
       help2.next = help3;
       help3.next = null ;   // null marks the end

       // Understand why this will create the above chain !
}

Constructing a linked list using explicit helper reference variables

  • We must use the first variable to reference to the first Node in the linked list: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       ...
       help1.next = help2;
       help2.next = help3;
       help3.next = null ;   // null marks the end

       first = help1;        // Done !
}

Constructing a linked list using explicit helper reference variables

  • If we remove the helper variables, you will see the linked list clearer: 

    public class Demo
{
   public static Node first; // Define the first variable

   public static void main(String[] args)
   {
       ...
       help1.next = help2;
       help2.next = help3;
       help3.next = null ;   // null marks the end

       first = help1;        // Done !
}

DEMO: demo/11-linked-list/01-create/Demo.java

Summary and insights

  • Nodes of a linked list can be created on demand

  • A linked list is created by chaining nodes together using the link field (which is usually named next)

  • Node addition and removal is archieved by manipulating the links in the appropriate nodes in the linked list !

  • A comment and an advice:

    • There are many variations possible

      So:

      • Do not memorize what to do

    • Understand what do you want to achieve:

      • How should the new set of links be ?

    • Then device the manipulation steps to achieve the result