Linked List

Linked List is a very commonly used linear data structure which consists of group of nodes in a sequence.Each node holds its own data and the address of the next node hence forming a chain like structure.A linked list is a linear data structure, in which the elements are not stored at contiguous memory locations. The elements in a linked list are linked using pointers as shown in the below image:

Why Linked List?
Arrays can be used to store linear data of similar types, but arrays have the following limitations.
1) The size of the arrays is fixed: So we must know the upper limit on the number of elements in advance. Also, generally, the allocated memory is equal to the upper limit irrespective of the usage.
2) Inserting a new element in an array of elements is expensive because the room has to be created for the new elements and to create room existing elements have to be shifted.

For example, in a system, if we maintain a sorted list of IDs in an array id[].

id[] = [1000, 1010, 1050, 2000, 2040].

And if we want to insert a new ID 1005, then to maintain the sorted order, we have to move all the elements after 1000 (excluding 1000).
Deletion is also expensive with arrays until unless some special techniques are used. For example, to delete 1010 in id[], everything after 1010 has to be moved.

Advantages over arrays
1) Dynamic size
2) Ease of insertion/deletion

Drawbacks:
1) Random access is not allowed. We have to access elements sequentially starting from the first node. So we cannot do binary search with linked lists efficiently with its default implementation.
2) Extra memory space for a pointer is required with each element of the list.
3) Not cache friendly. Since array elements are contiguous locations, there is locality of reference which is not there in case of linked lists.

Representation:
A linked list is represented by a pointer to the first node of the linked list. The first node is called the head. If the linked list is empty, then the value of the head is NULL.
Each node in a list consists of at least two parts:
1) data
2) Pointer (Or Reference) to the next node
In C, we can represent a node using structures. Below is an example of a linked list node with integer data.
In Java or C#, LinkedList can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

Example in Java

class Link

{

    public int rollNo;

    public String name;

    public Link next;

    public Link(int rNo,String n)

    {

        rollNo=rNo;

        name=n;

    }

    public void displayLink()

    {

        System.out.print("{"+rollNo+","+name+"}");

    }

}

The LinkList Class

The LinkList class contains only one element called as first which is used to maintain the address of first link in the list. Since in order to process the list, it’s mandatory to know the starting address of the list. Using the first link address all the Links can be processed very easily.

Basic operations on Single Linked lists

·    Inserting an item at the beginning of the list.

·    Deleting the item at the beginning of the list.

·    Display the contents of the entire list.

Advanced operations of Single Linked lists

·    Search for a given element in the list.

·    Delete a specified element form the list.

class LinkList

{

                private Link first;

                public void LinkList( )

{

                                first = null;

}

public boolean isEmpty( )

{

                                return(first==null);

}

public void insertFirst( ----)

{              

                ======

}

public Link deleteFirst()

{

                =====

}

public  void displayList()

{              

                ======

}

}

Here LinkList() constructor initializes the List by assigning null to first, since when the list is created it is empty.

The isEmpty() method returns true only when the list is empty otherwise false is returned

insertFirst() methods inserts an element at the beginning of the list

deleteFirst() methods deletes an element from the beginning of the list and displayList displays the entire linked list