Introduction

In this article, I am going to discuss one of the most important Data Structures- Linked List.

I will be explaining about

What is a Linked List?

Linked List is a linear data structure which consists of a group of nodes in a sequence. Each node contains two parts.

The first node of a Linked List is referenced by a pointer called Head

Advantages of Linked List

Types of Linked List

Creating a Linked List

The node of a singly linked list contains a data part and a link part. The link will contain the address of next node and is initialized to null. So, we will create class definition of node for singly linked list as follows -

internal class Node {  
    internal int data;  
    internal Node next;  
    public Node(int d) {  
        data = d;  
        next = null;  
    }  
}

The node for a Doubly Linked list will contain one data part and two link parts - previous link and next link. Hence, we create a class definition of a node for the doubly linked list as shown below.

internal class DNode {  
    internal int data;  
    internal DNode prev;  
    internal DNode next;  
    public DNode(int d) {  
        data = d;  
        prev = null;  
        next = null;  
    }  
}

Now, our node has been created, so, we will create a linked list class now. When a new Linked List is instantiated, it just has the head, which is Null.The SinglyLinkedList class will contain nodes of type Node class. Hence, SinglyLinkedList class definition will look like below.

internal class SingleLinkedList {  
    internal Node head;  
}

The DoublyLinkedList class will contain nodes of type DNode class. Hence, DoublyLinkedList class will look like this.

internal class DoubleLinkedList {  
    internal DNode head;  
} 

Various operations on Linked list

Insert data at front of the Linked List 

internal void InsertFront(SingleLinkedList singlyList, int new_data) {    
    Node new_node = new Node(new_data);    
    new_node.next = singlyList.head;    
    singlyList.head = new_node;    
} 

To insert the data at front of the doubly linked list, we have to follow one extra step .i.e point the previous pointer of head node to the new node. So, the method will look like this.

internal void InsertFront(DoubleLinkedList doubleLinkedList, int data) {  
    DNode newNode = new DNode(data);  
    newNode.next = doubleLinkedList.head;  
    newNode.prev = null;  
    if (doubleLinkedList.head != null) {  
        doubleLinkedList.head.prev = newNode;  
    }  
    doubleLinkedList.head = newNode;  
}

Insert data at the end of Linked List

internal void InsertLast(SingleLinkedList singlyList, int new_data)    
{    
    Node new_node = new Node(new_data);    
    if (singlyList.head == null) {    
        singlyList.head = new_node;    
        return;    
    }    
    Node lastNode = GetLastNode(singlyList);    
    lastNode.next = new_node;    
}

To insert the data at the end of a doubly linked list, we have to follow one extra step; .i.e., point previous pointer of new node to the last node.so the method will look like this.

internal void InsertLast(DoubleLinkedList doubleLinkedList, int data) {  
    DNode newNode = new DNode(data);  
    if (doubleLinkedList.head == null) {  
        newNode.prev = null;  
        doubleLinkedList.head = newNode;  
        return;  
    }  
    DNode lastNode = GetLastNode(doubleLinkedList);  
    lastNode.next = newNode;  
    newNode.prev = lastNode;  
}

The last node will be the one with its next pointing to null. Hence we will traverse the list until we find the node with next as null and return that node as last node. Therefore the method to get the last node will be

internal Node GetLastNode(SingleLinkedList singlyList) {  
    Node temp = singlyList.head;  
    while (temp.next != null) {  
        temp = temp.next;  
    }  
    return temp;  
}

In the above mentioned method, pass doubleLinkedList object to get last node for Doubly Linked List.

Insert data after a given node of Linked List

So the method for singly Linked List will look like this,

internal void InsertAfter(Node prev_node, int new_data)  
{  
    if (prev_node == null) {  
        Console.WriteLine("The given previous node Cannot be null");  
        return;  
    }  
    Node new_node = new Node(new_data);  
    new_node.next = prev_node.next;  
    prev_node.next = new_node;  
}

To perform this operation on doubly linked list we need to follow two extra steps

  1. Set the previous of new node to given node.

  2. Set the previous of the next node of given node to the new node.

So, the method for Doubly Linked List will look like this.

internal void InsertAfter(DNode prev_node, int data)  
{  
    if (prev_node == null) {  
        Console.WriteLine("The given prevoius node cannot be null");  
        return;  
    }  
    DNode newNode = new DNode(data);  
    newNode.next = prev_node.next;  
    prev_node.next = newNode;  
    newNode.prev = prev_node;  
    if (newNode.next != null) {  
        newNode.next.prev = newNode;  
    }  
}

Delete a node from Linked List using a given key value

So, the method for singly linked list will look like this,

internal void DeleteNodebyKey(SingleLinkedList singlyList, int key)  
{  
    Node temp = singlyList.head;  
    Node prev = null;  
    if (temp != null && temp.data == key) {  
        singlyList.head = temp.next;  
        return;  
    }  
    while (temp != null && temp.data != key) {  
        prev = temp;  
        temp = temp.next;  
    }  
    if (temp == null) {  
        return;  
    }  
    prev.next = temp.next;  
}

To perform this operation on doubly linked list we don't need any extra pointer for previous node as Doubly linked list already have a pointer to previous node.so the delete method will be,

internal void DeleteNodebyKey(DoubleLinkedList doubleLinkedList, int key)  
{  
    DNode temp = doubleLinkedList.head;  
    if (temp != null && temp.data == key) {  
        doubleLinkedList.head = temp.next;  
        doubleLinkedList.head.prev = null;  
        return;  
    }  
    while (temp != null && temp.data != key) {  
        temp = temp.next;  
    }  
    if (temp == null) {  
        return;  
    }  
    if (temp.next != null) {  
        temp.next.prev = temp.prev;  
    }  
    if (temp.prev != null) {  
        temp.prev.next = temp.next;  
    }  
}

Reverse a Singly Linked list

This is one of the most famous interview questions. We need to reverse the links of each node to point to its previous node, and the last node should be the head node.This can be achieved by iterative as well as recursive methods. Here I am explaining the iterative method.

The method will look like this,

public void ReverseLinkedList(SingleLinkedList singlyList)  
{  
    Node prev = null;  
    Node current = singlyList.head;  
    Node temp = null;  
    while (current != null) {  
        temp = current.next;  
        current.next = prev;  
        prev = current;  
        current = temp;  
    }  
    singlyList.head = prev;  
}

Conclusion

We have implemented Singly Linked list and Doubly Linked list using C#. We have also performed various operations on them. Please refer to the attached code for better understanding. You will find a few more methods in the attached code such as finding middle element and searching a linked list.

Please give your valuable feedback in the comment section.

See Also

Here are recommended articles on collections:

  1. C# Dictionary
  2. C# Queue
  3. C# Stack
  4. C# List
  5. C# Arrays
  6. C# HashTable
  7. C# StringCollection
  8. C# ArrayList
  9. C# HashSet
  10. C# LinkedList