Insertion in Linked List

We have introduced Linked Lists in the previous post. We also created a simple linked list with 3 nodes and discussed linked list traversal.
All programs discussed in this post consider the following representations of the linked list. 

C++

// A linked list node
class Node
{
    public:
    int data;
    Node *next;
};
// This code is contributed by rathbhupendra

C

// A linked list node
struct Node
{
  int data;
  struct Node *next;
};

Java

// Linked List Class
class LinkedList
{
    Node head;  // head of list

    /* Node Class */
    class Node
    {
        int data;
        Node next;

        // Constructor to create a new node
        Node(int d) {data = d; next = null; }
    }
}

Python3

# Node class
class Node:

    # Function to initialize the node object
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null

# Linked List class
class LinkedList:

    # Function to initialize the Linked List object
    def __init__(self):
        self.head = None

C#

/* Linked list Node*/
public class Node
{
    public int data;
    public Node next;
    public Node(int d) {data = d; next = null; }
}

Javascript

<script>
// Linked List Class
    var head; // head of list

    /* Node Class */
    class Node {

        // Constructor to create a new node
        constructor(d) {
            this.data = d;
            this.next = null;
        }
    }
// This code is contributed by todaysgaurav
</script>

DSA Self-paced course

In this post, methods to insert a new node in the linked list are discussed. A node can be added in three ways 

• At the front of the linked list  
• After a given node. 
• At the end of the linked list.

Add a node at the front: (4 steps process)

Approach: The new node is always added before the head of the given Linked List. And newly added node becomes the new head of the Linked List. For example, if the given Linked List is 10->15->20->25 and we add an item 5 at the front, then the Linked List becomes 5->10->15->20->25. Let us call the function that adds at the front of the list is push(). The push() must receive a pointer to the head pointer because the push must change the head pointer to point to the new node (See this) 

Following are the 4 steps to add a node at the front.

C++

// Given a reference (pointer to pointer)
// to the head of a list and an int,
// inserts a new node on the front of
// the list.
void push(Node** head_ref, int new_data)
{

    // 1. allocate node
    Node* new_node = new Node();

    // 2. put in the data
    new_node->data = new_data;

    // 3. Make next of new node as head
    new_node->next = (*head_ref);

    // 4. Move the head to point to
    // the new node
    (*head_ref) = new_node;
}

// This code is contributed by rathbhupendra

C

/* Given a reference (pointer to pointer) to the head of a list
   and an int,  inserts a new node on the front of the list. */
void push(struct Node** head_ref, int new_data)
{
    /* 1. allocate node */
    struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));

    /* 2. put in the data  */
    new_node->data  = new_data;

    /* 3. Make next of new node as head */
    new_node->next = (*head_ref);

    /* 4. move the head to point to the new node */
    (*head_ref)    = new_node;
}

Java

/* This function is in LinkedList class. Inserts a
   new Node at front of the list. This method is
   defined inside LinkedList class shown above */
public void push(int new_data)
{
    /* 1 & 2: Allocate the Node &
              Put in the data*/
    Node new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

Python3

# This function is in LinkedList class
# Function to insert a new node at the beginning
def push(self, new_data):

    # 1 & 2: Allocate the Node &
    #        Put in the data
    new_node = Node(new_data)

    # 3. Make next of new Node as head
    new_node.next = self.head

    # 4. Move the head to point to new Node
    self.head = new_node

C#

/* Inserts a new Node at front of the list. */
public void push(int new_data)
{
    /* 1 & 2: Allocate the Node &
               Put in the data*/
    Node new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

Javascript

<script>

/* This function is in LinkedList class. Inserts a
   new Node at front of the list. This method is
   defined inside LinkedList class shown above */

 function push(new_data)
{
    /* 1 & 2: Allocate the Node &
              Put in the data*/
    var new_node = new Node(new_data);

    /* 3. Make next of new Node as head */
    new_node.next = head;

    /* 4. Move the head to point to new Node */
    head = new_node;
}

// This code contributed by Rajput-Ji

</script>

Complexity Analysis:

• Time Complexity: O(1), We have a pointer to the head and we can directly attach a node and change the pointer. So the Time complexity of inserting a node at the head position is O(1) as it does a constant amount of work.
• Auxiliary Space: O(1)

Add a node after a given node: (5 steps process)

Approach: We are given a pointer to a node, and the new node is inserted after the given node.

Follow the steps to add a node after a given node:

• Firstly, check if the given previous node is NULL or not.
• Then, allocate a new node and
• Assign the data to the new node
• And then make the next of new node as the next of previous node. 
• Finally, move the next of the previous node as a new node.

C++

// Given a node prev_node, insert a
// new node after the given
// prev_node
void insertAfter(Node* prev_node, int new_data)
{

    // 1. Check if the given prev_node is NULL
    if (prev_node == NULL) {
        cout << "The given previous node cannot be NULL";
        return;
    }

    // 2. Allocate new node
    Node* new_node = new Node();

    // 3. Put in the data
    new_node->data = new_data;

    // 4. Make next of new node as
    // next of prev_node
    new_node->next = prev_node->next;

    // 5. move the next of prev_node
    // as new_node
    prev_node->next = new_node;
}

// This code is contributed by anmolgautam818,
// arkajyotibasak

C

/* Given a node prev_node, insert a new node after the given
prev_node */
void insertAfter(struct Node* prev_node, int new_data)
{
    /*1. check if the given prev_node is NULL */
    if (prev_node == NULL) {
        printf("the given previous node cannot be NULL");
        return;
    }

    /* 2. allocate new node */
    struct Node* new_node
        = (struct Node*)malloc(sizeof(struct Node));

    /* 3. put in the data */
    new_node->data = new_data;

    /* 4. Make next of new node as next of prev_node */
    new_node->next = prev_node->next;

    /* 5. move the next of prev_node as new_node */
    prev_node->next = new_node;
}

Java

/* This function is in LinkedList class.
Inserts a new node after the given prev_node. This method is
defined inside LinkedList class shown above */
public void insertAfter(Node prev_node, int new_data)
{
    /* 1. Check if the given Node is null */
    if (prev_node == null) {
        System.out.println(
            "The given previous node cannot be null");
        return;
    }

    /* 2. Allocate the Node &
    3. Put in the data*/
    Node new_node = new Node(new_data);

    /* 4. Make next of new Node as next of prev_node */
    new_node.next = prev_node.next;

    /* 5. make next of prev_node as new_node */
    prev_node.next = new_node;
}

Python3

# This function is in LinkedList class.
# Inserts a new node after the given prev_node. This method is
# defined inside LinkedList class shown above */


def insertAfter(self, prev_node, new_data):

    # 1. check if the given prev_node exists
    if prev_node is None:
        print("The given previous node must inLinkedList.")
        return

    # 2. Create new node &
    # 3. Put in the data
    new_node = Node(new_data)

    # 4. Make next of new Node as next of prev_node
    new_node.next = prev_node.next

    # 5. make next of prev_node as new_node
    prev_node.next = new_node

C#

/* Inserts a new node after the given prev_node. */
public void insertAfter(Node prev_node, int new_data)
{
    /* 1. Check if the given Node is null */
    if (prev_node == null) {
        Console.WriteLine("The given previous node"
                          + " cannot be null");
        return;
    }

    /* 2 & 3: Allocate the Node &
            Put in the data*/
    Node new_node = new Node(new_data);

    /* 4. Make next of new Node as
                next of prev_node */
    new_node.next = prev_node.next;

    /* 5. make next of prev_node
                    as new_node */
    prev_node.next = new_node;
}

Javascript

<script>

/* This function is in LinkedList class.
Inserts a new node after the given prev_node. This method is
defined inside LinkedList class shown above */
function insertAfter(prev_node , new_data)
{

    /* 1. Check if the given Node is null */
    if (prev_node == null)
    {
        document.write("The given previous node cannot be null");
        return;
    }

    /* 2. Allocate the Node &
    3. Put in the data*/
    var new_node = new Node(new_data);

    /* 4. Make next of new Node as next of prev_node */
    new_node.next = prev_node.next;

    /* 5. make next of prev_node as new_node */
    prev_node.next = new_node;
}

// This code is contributed by aashish1995

</script>

Complexity Analysis: 

Time complexity: O(1), since prev_node is already given as argument in a method, no need to iterate over list to find prev_node
Auxiliary Space: O(1) since using constant space to modify pointers

Add a node at the end: (6 steps process)

• The new node is always added after the last node of the given Linked List. For example if the given Linked List is 5->10->15->20->25 and we add an item 30 at the end, then the Linked List becomes 5->10->15->20->25->30. 
• Since a Linked List is typically represented by the head of it, we have to traverse the list till the end and then change the next to last node to a new node.
  

Following are the 6 steps to add a node at the end.

C++

// Given a reference (pointer to pointer) to the head
// of a list and an int, appends a new node at the end
void append(Node** head_ref, int new_data)
{

    // 1. allocate node
    Node* new_node = new Node();

    // Used in step 5
    Node *last = *head_ref;

    // 2. Put in the data
    new_node->data = new_data;

    // 3. This new node is going to be
    // the last node, so make next of
    // it as NULL
    new_node->next = NULL;

    // 4. If the Linked List is empty,
    // then make the new node as head
    if (*head_ref == NULL)
    {
        *head_ref = new_node;
        return;
    }

    // 5. Else traverse till the last node
    while (last->next != NULL)
    {
        last = last->next;
    }

    // 6. Change the next of last node
    last->next = new_node;
    return;
}

// This code is contributed by anmolgautam818, arkajyotibasak

C

/* Given a reference (pointer to pointer) to the head
   of a list and an int, appends a new node at the end  */
void append(struct Node** head_ref, int new_data)
{
    /* 1. allocate node */
    struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));

    struct Node *last = *head_ref;  /* used in step 5*/

    /* 2. put in the data  */
    new_node->data  = new_data;

    /* 3. This new node is going to be the last node, so make next
          of it as NULL*/
    new_node->next = NULL;

    /* 4. If the Linked List is empty, then make the new node as head */
    if (*head_ref == NULL)
    {
       *head_ref = new_node;
       return;
    }

    /* 5. Else traverse till the last node */
    while (last->next != NULL)
        last = last->next;

    /* 6. Change the next of last node */
    last->next = new_node;
    return;
}

Java

/* Appends a new node at the end.  This method is
   defined inside LinkedList class shown above */
public void append(int new_data)
{
    /* 1. Allocate the Node &
       2. Put in the data
       3. Set next as null */
    Node new_node = new Node(new_data);

    /* 4. If the Linked List is empty, then make the
           new node as head */
    if (head == null)
    {
        head = new Node(new_data);
        return;
    }

    /* 4. This new node is going to be the last node, so
         make next of it as null */
    new_node.next = null;

    /* 5. Else traverse till the last node */
    Node last = head;
    while (last.next != null)
        last = last.next;

    /* 6. Change the next of last node */
    last.next = new_node;
    return;
}

Python3

# This function is defined in Linked List class
# Appends a new node at the end.  This method is
# defined inside LinkedList class shown above


def append(self, new_data):

        # 1. Create a new node
        # 2. Put in the data
        # 3. Set next as None
        new_node = Node(new_data)

        # 4. If the Linked List is empty, then make the
        #    new node as head
        if self.head is None:
            self.head = new_node
            return

        # 5. Else traverse till the last node
        last = self.head
        while (last.next):
            last = last.next

        # 6. Change the next of last node
        last.next = new_node

C#

/* Appends a new node at the end. This method is
defined inside LinkedList class shown above */
public void append(int new_data)
{
    /* 1. Allocate the Node &
    2. Put in the data
    3. Set next as null */
    Node new_node = new Node(new_data);

    /* 4. If the Linked List is empty,
       then make the new node as head */
    if (head == null)
    {
        head = new Node(new_data);
        return;
    }

    /* 4. This new node is going to be
    the last node, so make next of it as null */
    new_node.next = null;

    /* 5. Else traverse till the last node */
    Node last = head;
    while (last.next != null)
        last = last.next;

    /* 6. Change the next of last node */
    last.next = new_node;
    return;
}

Javascript

<script>

/* Appends a new node at the end.  This method is
   defined inside LinkedList class shown above */
 function append(new_data)
{
    /* 1. Allocate the Node &
       2. Put in the data
       3. Set next as null */
    var new_node = new Node(new_data);

    /* 4. If the Linked List is empty, then make the
           new node as head */
    if (head == null)
    {
        head = new Node(new_data);
        return;
    }

    /* 4. This new node is going to be the last node, so
         make next of it as null */
    new_node.next = null;

    /* 5. Else traverse till the last node */
    var last = head;
    while (last.next != null)
        last = last.next;

    /* 6. Change the next of last node */
    last.next = new_node;
    return;
}

// This code contributed by aashish1995
</script>

Complexity Analysis:

• Time complexity: O(N), where N is the number of nodes in the linked list. Since there is a loop from head to end, the function does O(n) work. 
  • This method can also be optimized to work in O(1) by keeping an extra pointer to the tail of the linked list/
• Auxiliary Space: O(1)

Following is a complete program that uses all of the above methods to create a linked list.

C++

// A complete working C++ program to
// demonstrate all insertion methods
// on Linked List
#include <bits/stdc++.h>
using namespace std;

// A linked list node
class Node
{
    public:
    int data;
    Node *next;
};

// Given a reference (pointer to pointer)
// to the head of a list and an int, inserts
// a new node on the front of the list.
void push(Node** head_ref, int new_data)
{

    // 1. allocate node
    Node* new_node = new Node();

    // 2. put in the data
    new_node->data = new_data;

    // 3. Make next of new node as head
    new_node->next = (*head_ref);

    // 4. move the head to point
    // to the new node
    (*head_ref) = new_node;
}

// Given a node prev_node, insert a new
// node after the given prev_node
void insertAfter(Node* prev_node, int new_data)
{
    // 1. check if the given prev_node
    // is NULL
    if (prev_node == NULL)
    {
        cout<<"The given previous node cannot be NULL";
        return;
    }

    // 2. allocate new node
    Node* new_node = new Node();

    // 3. put in the data
    new_node->data = new_data;

    // 4. Make next of new node
    // as next of prev_node
    new_node->next = prev_node->next;

    // 5. move the next of prev_node
    // as new_node
    prev_node->next = new_node;
}

// Given a reference (pointer to pointer)
// to the head of a list and an int,
// appends a new node at the end
void append(Node** head_ref, int new_data)
{

    // 1. allocate node
    Node* new_node = new Node();

    //used in step 5
    Node *last = *head_ref;

    // 2. put in the data
    new_node->data = new_data;

    /* 3. This new node is going to be
    the last node, so make next of
    it as NULL*/
    new_node->next = NULL;

    /* 4. If the Linked List is empty,
    then make the new node as head */
    if (*head_ref == NULL)
    {
        *head_ref = new_node;
        return;
    }

    /* 5. Else traverse till the last node */
    while (last->next != NULL)
    {
        last = last->next;
    }

    /* 6. Change the next of last node */
    last->next = new_node;
    return;
}

// This function prints contents of
// linked list starting from head
void printList(Node *node)
{
    while (node != NULL)
    {
        cout<<" "<<node->data;
        node = node->next;
    }
}

// Driver code
int main()
{

    // Start with the empty list
    Node* head = NULL;

    // Insert 6. So linked list becomes 6->NULL
    append(&head, 6);

    // Insert 7 at the beginning.
    // So linked list becomes 7->6->NULL
    push(&head, 7);

    // Insert 1 at the beginning.
    // So linked list becomes 1->7->6->NULL
    push(&head, 1);

    // Insert 4 at the end. So
    // linked list becomes 1->7->6->4->NULL
    append(&head, 4);

    // Insert 8, after 7. So linked
    // list becomes 1->7->8->6->4->NULL
    insertAfter(head->next, 8);

    cout<<"Created Linked list is: ";
    printList(head);

    return 0;
}
// This code is contributed by rathbhupendra, arkajyotibasak

C

// A complete working C program to demonstrate all insertion methods
// on Linked List
#include <stdio.h>
#include <stdlib.h>

// A linked list node
struct Node
{
  int data;
  struct Node *next;
};

/* Given a reference (pointer to pointer) to the head of a list and
   an int, inserts a new node on the front of the list. */
void push(struct Node** head_ref, int new_data)
{
    /* 1. allocate node */
    struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));

    /* 2. put in the data  */
    new_node->data  = new_data;

    /* 3. Make next of new node as head */
    new_node->next = (*head_ref);

    /* 4. move the head to point to the new node */
    (*head_ref)    = new_node;
}

/* Given a node prev_node, insert a new node after the given
   prev_node */
void insertAfter(struct Node* prev_node, int new_data)
{
    /*1. check if the given prev_node is NULL */
    if (prev_node == NULL)
    {
      printf("the given previous node cannot be NULL");
      return;
    }

    /* 2. allocate new node */
    struct Node* new_node =(struct Node*) malloc(sizeof(struct Node));

    /* 3. put in the data  */
    new_node->data  = new_data;

    /* 4. Make next of new node as next of prev_node */
    new_node->next = prev_node->next;

    /* 5. move the next of prev_node as new_node */
    prev_node->next = new_node;
}

/* Given a reference (pointer to pointer) to the head
   of a list and an int, appends a new node at the end  */
void append(struct Node** head_ref, int new_data)
{
    /* 1. allocate node */
    struct Node* new_node = (struct Node*) malloc(sizeof(struct Node));

    struct Node *last = *head_ref;  /* used in step 5*/

    /* 2. put in the data  */
    new_node->data  = new_data;

    /* 3. This new node is going to be the last node, so make next of
          it as NULL*/
    new_node->next = NULL;

    /* 4. If the Linked List is empty, then make the new node as head */
    if (*head_ref == NULL)
    {
       *head_ref = new_node;
       return;
    }

    /* 5. Else traverse till the last node */
    while (last->next != NULL)
        last = last->next;

    /* 6. Change the next of last node */
    last->next = new_node;
    return;
}

// This function prints contents of linked list starting from head
void printList(struct Node *node)
{
  while (node != NULL)
  {
     printf(" %d ", node->data);
     node = node->next;
  }
}

/* Driver program to test above functions*/
int main()
{
  /* Start with the empty list */
  struct Node* head = NULL;

  // Insert 6.  So linked list becomes 6->NULL
  append(&head, 6);

  // Insert 7 at the beginning. So linked list becomes 7->6->NULL
  push(&head, 7);

  // Insert 1 at the beginning. So linked list becomes 1->7->6->NULL
  push(&head, 1);

  // Insert 4 at the end. So linked list becomes 1->7->6->4->NULL
  append(&head, 4);

  // Insert 8, after 7. So linked list becomes 1->7->8->6->4->NULL
  insertAfter(head->next, 8);

  printf("\n Created Linked list is: ");
  printList(head);

  return 0;
}

Java

// A complete working Java program to demonstrate all insertion methods
// on linked list
import java.io.*;
public class LinkedList
{
    Node head;  // head of list

    /* Linked list Node*/
    class Node
    {
        int data;
        Node next;
        Node(int d) {data = d; next = null; }
    }

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Inserts a new node after the given prev_node. */
    public void insertAfter(Node prev_node, int new_data)
    {
        /* 1. Check if the given Node is null */
        if (prev_node == null)
        {
            System.out.println("The given previous node cannot be null");
            return;
        }

        /* 2 & 3: Allocate the Node &
                  Put in the data*/
        Node new_node = new Node(new_data);

        /* 4. Make next of new Node as next of prev_node */
        new_node.next = prev_node.next;

        /* 5. make next of prev_node as new_node */
        prev_node.next = new_node;
    }

    /* Appends a new node at the end.  This method is
       defined inside LinkedList class shown above */
    public void append(int new_data)
    {
        /* 1. Allocate the Node &
           2. Put in the data
           3. Set next as null */
        Node new_node = new Node(new_data);

        /* 4. If the Linked List is empty, then make the
              new node as head */
        if (head == null)
        {
            head = new Node(new_data);
            return;
        }

        /* 4. This new node is going to be the last node, so
              make next of it as null */
        new_node.next = null;

        /* 5. Else traverse till the last node */
        Node last = head;
        while (last.next != null)
            last = last.next;

        /* 6. Change the next of last node */
        last.next = new_node;
        return;
    }

    /* This function prints contents of linked list starting from
        the given node */
    public void printList()
    {
        Node tnode = head;
        while (tnode != null)
        {
            System.out.print(tnode.data+" ");
            tnode = tnode.next;
        }
    }

    /* Driver program to test above functions. Ideally this function
       should be in a separate user class.  It is kept here to keep
       code compact */
    public static void main(String[] args)
    {
        /* Start with the empty list */
        LinkedList llist = new LinkedList();

        // Insert 6.  So linked list becomes 6->NUllist
        llist.append(6);

        // Insert 7 at the beginning. So linked list becomes
        // 7->6->NUllist
        llist.push(7);

        // Insert 1 at the beginning. So linked list becomes
        // 1->7->6->NUllist
        llist.push(1);

        // Insert 4 at the end. So linked list becomes
        // 1->7->6->4->NUllist
        llist.append(4);

        // Insert 8, after 7. So linked list becomes
        // 1->7->8->6->4->NUllist
        llist.insertAfter(llist.head.next, 8);

        System.out.println("\nCreated Linked list is: ");
        llist.printList();
    }
}
// This code is contributed by Rajat Mishra

Python3

# A complete working Python program to demonstrate all
# insertion methods of linked list

# Node class
class Node:

    # Function to initialise the node object
    def __init__(self, data):
        self.data = data  # Assign data
        self.next = None  # Initialize next as null


# Linked List class contains a Node object
class LinkedList:

    # Function to initialize head
    def __init__(self):
        self.head = None


    # Function to insert a new node at the beginning
    def push(self, new_data):

        # 1 & 2: Allocate the Node &
        #        Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node


    # This function is in LinkedList class. Inserts a
    # new node after the given prev_node. This method is
    # defined inside LinkedList class shown above */
    def insertAfter(self, prev_node, new_data):

        # 1. check if the given prev_node exists
        if prev_node is None:
            print("The given previous node must inLinkedList.")
            return

        #  2. create new node &
        #      Put in the data
        new_node = Node(new_data)

        # 4. Make next of new Node as next of prev_node
        new_node.next = prev_node.next

        # 5. make next of prev_node as new_node
        prev_node.next = new_node


    # This function is defined in Linked List class
    # Appends a new node at the end.  This method is
    # defined inside LinkedList class shown above */
    def append(self, new_data):

        # 1. Create a new node
        # 2. Put in the data
        # 3. Set next as None
        new_node = Node(new_data)

        # 4. If the Linked List is empty, then make the
        #    new node as head
        if self.head is None:
            self.head = new_node
            return

        # 5. Else traverse till the last node
        last = self.head
        while (last.next):
            last = last.next

        # 6. Change the next of last node
        last.next =  new_node


    # Utility function to print the linked list
    def printList(self):
        temp = self.head
        while (temp):
            print(temp.data,end=" ")
            temp = temp.next



# Code execution starts here
if __name__=='__main__':

    # Start with the empty list
    llist = LinkedList()

    # Insert 6.  So linked list becomes 6->None
    llist.append(6)

    # Insert 7 at the beginning. So linked list becomes 7->6->None
    llist.push(7);

    # Insert 1 at the beginning. So linked list becomes 1->7->6->None
    llist.push(1);

    # Insert 4 at the end. So linked list becomes 1->7->6->4->None
    llist.append(4)

    # Insert 8, after 7. So linked list becomes 1 -> 7-> 8-> 6-> 4-> None
    llist.insertAfter(llist.head.next, 8)

    print('Created linked list is: ')
    llist.printList()

# This code is contributed by Manikantan Narasimhan

C#

// A complete working C# program to demonstrate
// all insertion methods on linked list
using System;

class GFG
{
    public Node head; // head of list

    /* Linked list Node*/
    public class Node
    {
        public int data;
        public Node next;
        public Node(int d) {data = d; next = null;}
    }

    /* Inserts a new Node at front of the list. */
    public void push(int new_data)
    {
        /* 1 & 2: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Inserts a new node after the given prev_node. */
    public void insertAfter(Node prev_node, int new_data)
    {
        /* 1. Check if the given Node is null */
        if (prev_node == null)
        {
            Console.WriteLine("The given previous" +
                              " node cannot be null");
            return;
        }

        /* 2 & 3: Allocate the Node &
                Put in the data*/
        Node new_node = new Node(new_data);

        /* 4. Make next of new Node as
              next of prev_node */
        new_node.next = prev_node.next;

        /* 5. make next of prev_node as new_node */
        prev_node.next = new_node;
    }

    /* Appends a new node at the end. This method is
    defined inside LinkedList class shown above */
    public void append(int new_data)
    {
        /* 1. Allocate the Node &
        2. Put in the data
        3. Set next as null */
        Node new_node = new Node(new_data);

        /* 4. If the Linked List is empty,
        then make the new node as head */
        if (head == null)
        {
            head = new Node(new_data);
            return;
        }

        /* 4. This new node is going to be the last node,
            so make next of it as null */
        new_node.next = null;

        /* 5. Else traverse till the last node */
        Node last = head;
        while (last.next != null)
            last = last.next;

        /* 6. Change the next of last node */
        last.next = new_node;
        return;
    }

    /* This function prints contents of linked list
    starting from the given node */
    public void printList()
    {
        Node tnode = head;
        while (tnode != null)
        {
            Console.Write(tnode.data + " ");
            tnode = tnode.next;
        }
    }

    // Driver Code
    public static void Main(String[] args)
    {
        /* Start with the empty list */
        GFG llist = new GFG();

        // Insert 6. So linked list becomes 6->NUllist
        llist.append(6);

        // Insert 7 at the beginning.
        // So linked list becomes 7->6->NUllist
        llist.push(7);

        // Insert 1 at the beginning.
        // So linked list becomes 1->7->6->NUllist
        llist.push(1);

        // Insert 4 at the end. So linked list becomes
        // 1->7->6->4->NUllist
        llist.append(4);

        // Insert 8, after 7. So linked list becomes
        // 1->7->8->6->4->NUllist
        llist.insertAfter(llist.head.next, 8);

        Console.Write("Created Linked list is: ");
        llist.printList();
    }
}

// This code is contributed by Rajput-Ji

Javascript

<script>

// A complete working javascript program
// to demonstrate all insertion methods
// on linked list
     var head; // head of list

    /* Linked list Node */
     class Node {
            constructor(val) {
                this.data = val;
                this.next = null;
            }
        }

    /* Inserts a new Node at front of the list. */
     function push(new_data) {
        /*
         * 1 & 2: Allocate the Node & Put in the data
         */
        var new_node = new Node(new_data);

        /* 3. Make next of new Node as head */
        new_node.next = head;

        /* 4. Move the head to point to new Node */
        head = new_node;
    }

    /* Inserts a new node after the given prev_node. */
     function insertAfter(prev_node , new_data) {
        /* 1. Check if the given Node is null */
        if (prev_node == null) {
            document.write(
            "The given previous node cannot be null"
            );
            return;
        }

        /*
         * 2 & 3: Allocate the Node & Put in the data
         */
         var new_node = new Node(new_data);

        /* 4. Make next of new Node as next of prev_node */
        new_node.next = prev_node.next;

        /* 5. make next of prev_node as new_node */
        prev_node.next = new_node;
    }

    /*
     * Appends a new node at the end.
       This method is defined inside LinkedList class
     * shown above
     */
     function append(new_data) {
        /*
         * 1. Allocate the Node & 2.
         Put in the data 3. Set next as null
         */
         var new_node = new Node(new_data);

        /*
         * 4. If the Linked List is empty,
         then make the new node as head
         */
        if (head == null) {
            head = new Node(new_data);
            return;
        }

        /*
         * 4. This new node is going to be the last node,
         so make next of it as null
         */
        new_node.next = null;

        /* 5. Else traverse till the last node */
        var last = head;
        while (last.next != null)
            last = last.next;

        /* 6. Change the next of last node */
        last.next = new_node;
        return;
    }

    /*
     * This function prints contents of linked list
     starting from the given node
     */
     function printList() {
var tnode = head;
        while (tnode != null) {
            document.write(tnode.data + " ");
            tnode = tnode.next;
        }
    }

    /*
     * Driver program to test above functions.
       Ideally this function should be in a
     * separate user class. It is kept here
     to keep code compact
     */

        /* Start with the empty list */

        // Insert 6. So linked list becomes 6->NUllist
        append(6);

        // Insert 7 at the beginning. So linked list becomes
        // 7->6->NUllist
        push(7);

        // Insert 1 at the beginning. So linked list becomes
        // 1->7->6->NUllist
        push(1);

        // Insert 4 at the end. So linked list becomes
        // 1->7->6->4->NUllist
        append(4);

        // Insert 8, after 7. So linked list becomes
        // 1->7->8->6->4->NUllist
        insertAfter(head.next, 8);

        document.write("\nCreated Linked list is: ");
        printList();

// This code contributed by gauravrajput1

</script>

Output

Created Linked list is:  1 7 8 6 4

Time Complexity: O(N)
Auxiliary Space: O(1)

An alternate method by using a constructor call:

• However, another method uses a constructor call inside the node class to minimize the memory allocation work.
• It also reduces the number of lines of code.

Below is the implementation of the above method:

C++

// Alternate method to declare the class
// in order to minimize the
// memory allocation work

#include <bits/stdc++.h>
using namespace std;

class node {
public:
    int data;
    node* next;

    // A constructor is called here
    node(int value)
    {

        // It automatically assigns the
        // value to the data
        data = value;

        // Next pointer is pointed to NULL
        next = NULL;
    }
};

// Function to insert an element
// at head position
void insertathead(node*& head, int val)
{
    node* n = new node(val);
    n->next = head;
    head = n;
}

// Function to insert a element
// at a specified position
void insertafter(node* head, int key, int val)
{
    node* n = new node(val);
    if (key == head->data) {
        n->next = head->next;
        head->next = n;
        return;
    }

    node* temp = head;
    while (temp->data != key) {
        temp = temp->next;
        if (temp == NULL) {
            return;
        }
    }
    n->next = temp->next;
    temp->next = n;
}

// Function to insert an
// element at the end
void insertattail(node*& head, int val)
{
    node* n = new node(val);
    if (head == NULL) {
        head = n;
        return;
    }

    node* temp = head;
    while (temp->next != NULL) {
        temp = temp->next;
    }
    temp->next = n;
}

// Function to print the
// singly linked list
void print(node*& head)
{
    node* temp = head;

    while (temp != NULL) {
        cout << temp->data << " -> ";
        temp = temp->next;
    }
    cout << "NULL" << endl;
}

// Main function
int main()
{

    // Declaring an empty linked list
    node* head = NULL;

    insertathead(head, 1);
    insertathead(head, 2);
    cout << "After insertion at head: ";
    print(head);
    cout << endl;

    insertattail(head, 4);
    insertattail(head, 5);
    cout << "After insertion at tail: ";
    print(head);
    cout << endl;

    insertafter(head, 1, 2);
    insertafter(head, 5, 6);
    cout << "After insertion at a given position: ";
    print(head);
    cout << endl;

    return 0;
}
// contributed by divyanshmishra101010

Java

// Alternate method to declare the class
// in order to minimize the
// memory allocation work

import java.io.*;

class node {
    int data;
    node next;

    // constructor call
    node(int value)
    {

        // It automatically assigns the value to the data
        data = value;

        // next pointer is pointed to null
        next = null;
    }
}

class GFG {

    // Declaring an empty linked list
    static node head = null;

    // Function to insert an element at head position
    static void insertathead(int val)
    {
        node n = new node(val);
        n.next = head;
        head = n;
    }

    // Function to insert a element at a specified position
    static void insertafter(int key, int val)
    {
        node n = new node(val);
        if (key == head.data) {
            n.next = head.next;
            head.next = n;
            return;
        }
        node temp = head;
        while (temp.data != key) {
            temp = temp.next;
            if (temp == null) {
                return;
            }
        }
        n.next = temp.next;
        temp.next = n;
    }

    // Function to insert an element at the end
    static void insertattail(int val)
    {
        node n = new node(val);
        if (head == null) {
            head = n;
            return;
        }
        node temp = head;
        while (temp.next != null) {
            temp = temp.next;
        }
        temp.next = n;
    }

    // Function to print the singly linked list
    static void print()
    {
        node temp = head;
        while (temp != null) {
            System.out.print(temp.data + " -> ");
            temp = temp.next;
        }
        System.out.println("NULL");
    }

    public static void main(String[] args)
    {

        insertathead(1);
        insertathead(2);
        System.out.print("After insertion at head: ");
        print();
        System.out.println();

        insertattail(4);
        insertattail(5);
        System.out.print("After insertion at tail: ");
        print();
        System.out.println();

        insertafter(1, 2);
        insertafter(5, 6);
        System.out.print(
            "After insertion at a given position: ");
        print();
        System.out.println();
    }
}

// This code is contributed by lokeshmvs21.

Python3

'''Alternate method to declare the class
in order to minimize the memory allocation work'''

# Node class
class Node:

    # A constructor is called here
    def __init__(self, data):
        self.data = data  # Automatically assign data
        self.next = None  # Initialize next pointer as null


# Linked List class contains a Node object
class LinkedList:

    # Function to initialize head
    def __init__(self):
        self.head = None

    '''This function is in LinkedList class.
    It inserts a new node at the beginning'''
    def insertAtHead(self, new_data):

        # 1 & 2: Allocate the Node &
        #        Put in the data
        new_node = Node(new_data)

        # 3. Make next of new Node as head
        new_node.next = self.head

        # 4. Move the head to point to new Node
        self.head = new_node

    '''This function is in LinkedList class. It inserts a
    new node after the given prev_data. This method is
    defined inside LinkedList class shown above'''
    def insertAfter(self, prev_data, new_data):

        # 1. check if the Linked List is empty or not
        if self.head is None:
            return

        # 2. Create new node & Put in the data
        new_node = Node(new_data)

        # 3. If prev_data is at the first position
        if (prev_data == self.head.data):
            new_node.next = self.head.next
            self.head.next = new_node
            return

        # 4. check if the given prev_data exists
        head = self.head
        while (head.data != prev_data):
            head = head.next
            if head == None:
                return
        new_node.next = head.next
        head.next = new_node

    '''This function is defined in Linked List class
    Appends a new node at the end. This method is
    defined inside LinkedList class shown above'''
    def insertattail(self, new_data):

        # 1. Create a new node
        # 2. Put in the data
        # 3. Set next as None
        new_node = Node(new_data)

        # 4. If the Linked List is empty, then make the
        #    new node as head
        if self.head is None:
            self.head = new_node
            return

        # 5. Else traverse till the last node
        last = self.head
        while (last.next):
            last = last.next

        # 6. Change the next of last node
        last.next = new_node

    # Utility function to print the linked list
    def printList(self):
        temp = self.head
        while (temp):
            print(temp.data, end=" -> ")
            temp = temp.next
        print("NULL")


# Code execution starts here
if __name__ == '__main__':

    # Start with the empty list
    llist = LinkedList()

    llist.insertAtHead(1)
    llist.insertAtHead(2)
    print("After insertion at head:", end=" ")
    llist.printList()
    print()

    llist.insertattail(4)
    llist.insertattail(5)
    print("After insertion at tail:", end=" ")
    llist.printList()
    print()

    llist.insertAfter(1, 2)
    llist.insertAfter(5, 6)
    print("After insertion at a given position:", end=" ")
    llist.printList()

# This code is contributed by Susobhan Akhuli

C#

using System;

// Alternate method to declare the class
// in order to minimize the
// memory allocation work
public class node {
  public int data;
  public node next;

  // constructor call
  public node(int value)
  {

    // It automatically assigns the value to the data
    data = value;

    // next pointer is pointed to null
    next = null;
  }
}

public class GFG {

  // Declaring an empty linked list
  public static node head = null;

  // Function to insert an element at head position
  public static void insertathead(int val)
  {
    node n = new node(val);
    n.next = head;
    head = n;
  }

  // Function to insert a element at a specified position
  public static void insertafter(int key, int val)
  {
    node n = new node(val);
    if (key == head.data) {
      n.next = head.next;
      head.next = n;
      return;
    }
    node temp = head;
    while (temp.data != key) {
      temp = temp.next;
      if (temp == null) {
        return;
      }
    }
    n.next = temp.next;
    temp.next = n;
  }

  // Function to insert an element at the end
  public static void insertattail(int val)
  {
    node n = new node(val);
    if (head == null) {
      head = n;
      return;
    }
    node temp = head;
    while (temp.next != null) {
      temp = temp.next;
    }
    temp.next = n;
  }

  // Function to print the singly linked list
  public static void print()
  {
    node temp = head;
    while (temp != null) {
      Console.Write(temp.data + " -> ");
      temp = temp.next;
    }
    Console.WriteLine("NULL");
  }

  static public void Main()
  {

    insertathead(1);
    insertathead(2);
    Console.Write("After insertion at head: ");
    print();
    Console.WriteLine();

    insertattail(4);
    insertattail(5);
    Console.Write("After insertion at tail: ");
    print();
    Console.WriteLine();

    insertafter(1, 2);
    insertafter(5, 6);
    Console.Write(
      "After insertion at a given position: ");
    print();
    Console.WriteLine();
  }
}

// This code is contributed by akashish__

Javascript

<script>

// Alternate method to declare the class
// in order to minimize the
// memory allocation work
     var head; // head of list

    /* Linked list Node */
     class Node {
            constructor(val) {
                this.data = val;
                this.next = null;
            }
        }

    // This function inserts a new Node at front of the list.
     function insertAtHead(new_data) {
        var new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }

    // This function inserts a new node after a specefic position
     function insertAfter(prev_data , new_data) {
        var new_node = new Node(new_data);
        if(prev_data == head.data){
            new_node.next = head.next;
            head.next = new_node;
        }

        var temp = head;
        while (temp.data != prev_data){
            temp = temp.next;
            if(temp == null)
                return;
        }
        new_node.next = temp.next
        temp.next = new_node;
    }

    // This function appends a new node at the end.
     function insertAtTail(new_data) {
         var new_node = new Node(new_data);
        if (head == null) {
            head = new Node(new_data);
            return;
        }

        new_node.next = null;

        var last = head;
        while (last.next != null)
            last = last.next;

        last.next = new_node;
        return;
    }

    // This function prints contents of linked list.
     function printList() {
        var tnode = head;
        while (tnode != null) {
            document.write(tnode.data + " -> ");
            tnode = tnode.next;
        }
        document.write("NULL");
    }

    // Driver program
    // Start with the empty list
    insertAtHead(1);
    insertAtHead(2);
    document.write("After insertion at head: ");
    printList();
    document.write("<br>");

    insertAtTail(4);
    insertAtTail(5);
    document.write("<br>After insertion at tail: ");
    printList();
    document.write("<br>");

    insertAfter(1, 2);
    insertAfter(5, 6);
    document.write("<br>After insertion at a given position: ");
    printList();
    document.write("<br>");

// This code contributed by Susobhan Akhuli

</script>

Output

After insertion at head: 2 -> 1 -> NULL

After insertion at tail: 2 -> 1 -> 4 -> 5 -> NULL

After insertion at a given position: 2 -> 1 -> 2 -> 4 -> 5 -> 6 -> NULL

Time Complexity: O(N)
Auxiliary Space: O(1)

You may like to try Practice MCQ Questions on Linked List
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