Introduction and Insertion in a Doubly Linked List

A Doubly Linked List (DLL) contains an extra pointer, typically called the previous pointer, together with the next pointer and data which are there in the singly linked list.

Prerequisites: Linked List Introduction, Inserting a node in Singly Linked List

Following is a representation of a DLL node:

C++

// Node of a doubly linked list
class Node {
public:
    int data;

    // Pointer to next node in DLL
    Node* next;

    // Pointer to previous node in DLL
    Node* prev;
};

// This code is contributed by shivanisinghss2110

C

// Node of a doubly linked list

struct Node {
    int data;

    // Pointer to next node in DLL
    struct Node* next;

    // Pointer to previous node in DLL
    struct Node* prev;
};

Java

// Class for Doubly Linked List

public class DLL {

    // Head of list
    Node head;

    // Doubly Linked list Node
    class Node {
        int data;
        Node prev;
        Node next;

        // Constructor to create a new node
        // next and prev is by default initialized as null
        Node(int d) { data = d; }
    }
}

Python3

# Node of a doubly linked list

class Node:
    def __init__(self, next=None, prev=None, data=None):

        # reference to next node in DLL
        self.next = next

        # reference to previous node in DLL
        self.prev = prev
        self.data = data

C#

// Class for Doubly Linked List

public class DLL {

    // head of list
    Node head;

    // Doubly Linked list Node
    public class Node {
        public int data;
        public Node prev;
        public Node next;

        // Constructor to create a new node
        // next and prev is by default initialized as null
        Node(int d) { data = d; }
    }
}

// This code contributed by gauravrajput1

Javascript

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

    /* Doubly Linked list Node */
     class Node {
        // Constructor to create a new node
            // next and prev is by default initialized as null
            constructor(val) {
                this.data = val;
                this.prev = null;
                this.next = null;
            }
        }


// This code contributed by gauravrajput1
</script>

Advantages of DLL over the singly linked list:

• A DLL can be traversed in both forward and backward directions. 
• The delete operation in DLL is more efficient if a pointer to the node to be deleted is given. 
• We can quickly insert a new node before a given node. 
• In a singly linked list, to delete a node, a pointer to the previous node is needed. To get this previous node, sometimes the list is traversed. In DLL, we can get the previous node using the previous pointer. 

Disadvantages of DLL over the singly linked list:

• Every node of DLL Requires extra space for a previous pointer. It is possible to implement DLL with a single pointer though (See this and this). 
• All operations require an extra pointer previous to be maintained. For example, in insertion, we need to modify previous pointers together with the next pointers. For example in the following functions for insertions at different positions, we need 1 or 2 extra steps to set the previous pointer.

Applications of DLL:

• It is used by web browsers for backward and forward navigation of web pages 
• LRU ( Least Recently Used ) / MRU ( Most Recently Used ) Cache are constructed using Doubly Linked Lists. 
• Used by various applications to maintain undo and redo functionalities. 
• In Operating Systems, a doubly linked list is maintained by thread scheduler to keep track of processes that are being executed at that time.

Insertion in DLL:

A node can be added in four ways:

• At the front of the DLL 
• After a given node. 
• At the end of the DLL 
• Before a given node.

1) Add a node at the front:

The new node is always added before the head of the given Linked List. And newly added node becomes the new head of DLL. For example, if the given Linked List is 1->0->1->5 and we add an item 5 at the front, then the Linked List becomes 5->1->0->1->5. Let us call the function that adds at the front of the list 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)

Below is the implementation of the 5 steps to insert a node at the front of the linked list:

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
    and previous as NULL */
    new_node->next = (*head_ref);
    new_node->prev = NULL;

    /* 4. change prev of head node to new node */
    if ((*head_ref) != NULL)
        (*head_ref)->prev = new_node;

    /* 5. move the head to point to the new node */
    (*head_ref) = new_node;
}

// This code is contributed by shivanisinghss2110

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 and previous as NULL
     */
    new_node->next = (*head_ref);
    new_node->prev = NULL;

    /* 4. change prev of head node to new node */
    if ((*head_ref) != NULL)
        (*head_ref)->prev = new_node;

    /* 5. move the head to point to the new node */
    (*head_ref) = new_node;
}

Java

// Adding a node at the front of the list
public void push(int new_data)
{
    /* 1. allocate node
     * 2. put in the data */
    Node new_Node = new Node(new_data);

    /* 3. Make next of new node as head and previous as NULL
     */
    new_Node.next = head;
    new_Node.prev = null;

    /* 4. change prev of head node to new node */
    if (head != null)
        head.prev = new_Node;

    /* 5. move the head to point to the new node */
    head = new_Node;
}

Python3

# Adding a node at the front of the list
def push(self, new_data):

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

    # 3. Make next of new node as head and previous as NULL
    new_node.next = self.head
    new_node.prev = None

    # 4. change prev of head node to new node
    if self.head is not None:
        self.head.prev = new_node

    # 5. move the head to point to the new node
    self.head = new_node

# This code is contributed by jatinreaper

C#

// Adding a node at the front of the list
public void push(int new_data)
{

    /* 1. allocate node
     * 2. put in the data */
    Node new_Node = new Node(new_data);

    /* 3. Make next of new node as head and previous as NULL
     */
    new_Node.next = head;
    new_Node.prev = null;

    /* 4. change prev of head node to new node */
    if (head != null)
        head.prev = new_Node;

    /* 5. move the head to point to the new node */
    head = new_Node;
}

// This code is contributed by aashish1995

Javascript

// Adding a node at the front of the list
function push(new_data)
{
    /* 1. allocate node
    * 2. put in the data */
    let new_Node = new Node(new_data);

    /* 3. Make next of new node as head and previous as NULL */
    new_Node.next = head;
    new_Node.prev = null;

    /* 4. change prev of head node to new node */
    if (head != null)
        head.prev = new_Node;

    /* 5. move the head to point to the new node */
    head = new_Node;
}

// This code is contributed by saurabh_jaiswal.

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

Note: Four steps of the above five steps are the same as the 4 steps used for inserting at the front in the singly linked list. The only extra step is to change the previous head.

2) Add a node after a given node:

We are given a pointer to a node as prev_node, and the new node is inserted after the given node.

Below is the implementation of the 7 steps to insert a node after a given node in the linked list:

C++

/* Given a node as prev_node, insert
a new node after the given 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. Make the next of prev_node as new_node */
    prev_node->next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node->prev = prev_node;

    /* 7. Change previous of new_node's next node */
    if (new_node->next != NULL)
        new_node->next->prev = new_node;
}

// This code is contributed by shivanisinghss2110.

C

/* Given a node as prev_node, insert a new node after the
 * given 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. Make the next of prev_node as new_node */
    prev_node->next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node->prev = prev_node;

    /* 7. Change previous of new_node's next node */
    if (new_node->next != NULL)
        new_node->next->prev = new_node;
}

Java

/* Given a node as prev_node, insert a new node after the
 * given node */
public void InsertAfter(Node prev_Node, int new_data)
{

    /*1. check if the given prev_node is NULL */
    if (prev_Node == null) {
        System.out.println(
            "The given previous node cannot be NULL ");
        return;
    }

    /* 2. allocate 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 the next of prev_node as new_node */
    prev_Node.next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node.prev = prev_Node;

    /* 7. Change previous of new_node's next node */
    if (new_node.next != null)
        new_node.next.prev = new_node;
}

Python3

# Given a node as prev_node, insert
# a new node after the given node


def insertAfter(self, prev_node, new_data):

    # 1. check if the given prev_node is NULL
    if prev_node is None:
        print("This node doesn't exist in DLL")
        return

    # 2. allocate node  & 3. put in the data
    new_node = Node(data=new_data)

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

    # 5. Make the next of prev_node as new_node
    prev_node.next = new_node

    # 6. Make prev_node as previous of new_node
    new_node.prev = prev_node

    # 7. Change previous of new_node's next node */
    if new_node.next is not None:
        new_node.next.prev = new_node

#  This code is contributed by jatinreaper

C#

/* Given a node as prev_node, insert a new node after the
 * given node */
public void InsertAfter(Node prev_Node, int new_data)
{

    /*1. check if the given prev_node is NULL */
    if (prev_Node == null) {
        Console.WriteLine(
            "The given previous node cannot be NULL ");
        return;
    }

    /* 2. allocate 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 the next of prev_node as new_node */
    prev_Node.next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node.prev = prev_Node;

    /* 7. Change previous of new_node's next node */
    if (new_node.next != null)
        new_node.next.prev = new_node;
}

// This code is contributed by aashish1995

Javascript

<script>

function InsertAfter(prev_Node,new_data)
{
        /*1. check if the given prev_node is NULL */
    if (prev_Node == null) {
        document.write("The given previous node cannot be NULL <br>");
        return;
    }

    /* 2. allocate node
    * 3. put in the data */
    let 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 the next of prev_node as new_node */
    prev_Node.next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node.prev = prev_Node;

    /* 7. Change previous of new_node's next node */
    if (new_node.next != null)
        new_node.next.prev = new_node;
}


// This code is contributed by unknown2108

</script>

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

Note: Five of the above steps step process are the same as the 5 steps used for inserting after a given node in the singly linked list. The two extra steps are needed to change the previous pointer of the new node and the previous pointer of the new node’s next node.

3) Add a node at the end:

The new node is always added after the last node of the given Linked List. For example, if the given DLL is 5->1->0->1->5->2 and we add item 30 at the end, then the DLL becomes 5->1->0->1->5->2->30. Since a Linked List is typically represented by its head of it, we have to traverse the list till the end and then change the next of last node to the new node.

Below is the implementation of the 7 steps to insert a node at the end of the linked list:

C++

/* Given a reference (pointer to pointer) to the head
of a DLL 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();

    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) {
        new_node->prev = 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;

    /* 7. Make last node as previous of new node */
    new_node->prev = last;

    return;
}

// This code is contributed by shivanisinghss2110

C

/* Given a reference (pointer to pointer) to the head
   of a DLL 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) {
        new_node->prev = 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;

    /* 7. Make last node as previous of new node */
    new_node->prev = last;

    return;
}

Java

// Add a node at the end of the list
void append(int new_data)
{
    /* 1. allocate node
     * 2. put in the data */
    Node new_node = new Node(new_data);

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

    /* 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 == null) {
        new_node.prev = null;
        head = 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;

    /* 7. Make last node as previous of new node */
    new_node.prev = last;
}

Python3

# Add a node at the end of the DLL
def append(self, new_data):

    # 1. allocate node 2. put in the data
    new_node = Node(data=new_data)
    last = self.head

    # 3. This new node is going to be the
    # last node, so make next of it as NULL
    new_node.next = None

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

    # 5. Else traverse till the last node
    while (last.next is not None):
        last = last.next

    # 6. Change the next of last node
    last.next = new_node
    # 7. Make last node as previous of new node */
    new_node.prev = last

#  This code is contributed by jatinreaper

C#

// Add a node at the end of the list
void append(int new_data)
{
    /* 1. allocate node
     * 2. put in the data */
    Node new_node = new Node(new_data);

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

    /* 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 == null) {
        new_node.prev = null;
        head = 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;

    /* 7. Make last node as previous of new node */
    new_node.prev = last;
}

// This code is contributed by shivanisinghss2110

Javascript

<script>
// Add a node at the end of the list
function append(new_data)
{
    /* 1. allocate node
     * 2. put in the data */
    var new_node = new Node(new_data);

    var last = head; /* used in step 5*/

    /* 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 == null) {
        new_node.prev = null;
        head = 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;

    /* 7. Make last node as previous of new node */
    new_node.prev = last;
}

// This code is contributed by Rajput-Ji
</script>

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

Note: Six of the above 7 steps are the same as the 6 steps used for inserting after a given node in the singly linked list. One extra step is needed to change the previous pointer of the new node.

4) Add a node before a given node:

Follow the below steps to solve the problem:

Let the pointer to this given node be next_node and the data of the new node be added as new_data. 

• Check if the next_node is NULL or not. If it’s NULL, return from the function because any new node can not be added before a NULL
• Allocate memory for the new node, let it be called new_node
• Set new_node->data = new_data
• Set the previous pointer of this new_node as the previous node of the next_node, new_node->prev = next_node->prev
• Set the previous pointer of the next_node as the new_node, next_node->prev = new_node
• Set the next pointer of this new_node as the next_node, new_node->next = next_node;
• If the previous node of the new_node is not NULL, then set the next pointer of this previous node as new_node, new_node->prev->next = new_node
• Else, if the prev of new_node is NULL, it will be the new head node. So, make (*head_ref) = new_node.

Following is the complete program to test the above functions:

C++

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

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

/* 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
    and previous as NULL */
    new_node->next = (*head_ref);
    new_node->prev = NULL;

    /* 4. change prev of head node to new node */
    if ((*head_ref) != NULL)
        (*head_ref)->prev = new_node;

    /* 5. move the head to point to the new node */
    (*head_ref) = new_node;
}

/* Given a node as prev_node, insert
a new node after the given 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. Make the next of prev_node as new_node */
    prev_node->next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node->prev = prev_node;

    /* 7. Change previous of new_node's next node */
    if (new_node->next != NULL)
        new_node->next->prev = new_node;
}

/* Given a reference (pointer to pointer) to the head
of a DLL 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();

    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) {
        new_node->prev = 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;

    /* 7. Make last node as previous of new node */
    new_node->prev = last;

    return;
}

// This function prints contents of
// linked list starting from the given node
void printList(Node* node)
{
    Node* last;
    cout << "\nTraversal in forward direction \n";
    while (node != NULL) {
        cout << node->data << " ";
        last = node;
        node = node->next;
    }

    cout << "\nTraversal in reverse direction \n";
    while (last != NULL) {
        cout << last->data << " ";
        last = last->prev;
    }
}

// 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 DLL is: ";
    printList(head);

    return 0;
}

// This is code is contributed by rathbhupendra

C

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

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

/* 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 and previous as NULL
     */
    new_node->next = (*head_ref);
    new_node->prev = NULL;

    /* 4. change prev of head node to new node */
    if ((*head_ref) != NULL)
        (*head_ref)->prev = new_node;

    /* 5. move the head to point to the new node */
    (*head_ref) = new_node;
}

/* Given a node as prev_node, insert a new node after the
 * given 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. Make the next of prev_node as new_node */
    prev_node->next = new_node;

    /* 6. Make prev_node as previous of new_node */
    new_node->prev = prev_node;

    /* 7. Change previous of new_node's next node */
    if (new_node->next != NULL)
        new_node->next->prev = new_node;
}

/* Given a reference (pointer to pointer) to the head
   of a DLL 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) {
        new_node->prev = 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;

    /* 7. Make last node as previous of new node */
    new_node->prev = last;

    return;
}

// This function prints contents of linked list starting
// from the given node
void printList(struct Node* node)
{
    struct Node* last;
    printf("\nTraversal in forward direction \n");
    while (node != NULL) {
        printf("%d ", node->data);
        last = node;
        node = node->next;
    }

    printf("\nTraversal in reverse direction \n");
    while (last != NULL) {
        printf("%d ", last->data);
        last = last->prev;
    }
}

// Driver code
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("Created DLL is: ");
    printList(head);

    getchar();
    return 0;
}

Java

// A complete working Java program to demonstrate all

/* Doubly Linked list Node*/
class Node {
  int data;
  Node prev;
  Node next;

  // Constructor to create a new node
  // next and prev is by default initialized as null
  Node(int d) {
    data = d;
    prev = null;
    next = null;
  }
}
// Class for Doubly Linked List
class DLL {
    static Node head = null; // head of list

    // Adding a node at the front of the list
    public static void push(int new_data)
    {
        /* 1. allocate node
         * 2. put in the data */
        Node new_Node = new Node(new_data);

        /* 3. Make next of new node as head and previous as
         * NULL */
        new_Node.next = head;

        /* 4. change prev of head node to new node */
        if (head != null)
            head.prev = new_Node;

        /* 5. move the head to point to the new node */
        head = new_Node;
    }

    // Add a node before the given node
    public static void InsertBefore(Node next_node, int new_data)
    {
        /*Check if the given nx_node is NULL*/
        if (next_node == null) {
            System.out.println(
                "The given next node can not be NULL");
            return;
        }

        // Allocate node, put in the data
        Node new_node = new Node(new_data);

        // Making prev of new node as prev of next node
        new_node.prev = next_node.prev;

        // Making prev of next node as new node
        next_node.prev = new_node;

        // Making next of new node as next node
        new_node.next = next_node;

        // Check if new node is added as head
        if (new_node.prev != null)
            new_node.prev.next = new_node;
        else
            head = new_node;
    }

    /* Given a node as prev_node, insert
    a new node after the given node */
    public static void InsertAfter(Node prev_Node, int new_data)
    {

        /*1. check if the given prev_node is NULL */
        if (prev_Node == null) {
            System.out.println(
                "The given previous node cannot be NULL ");
            return;
        }

        /* 2. allocate 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 the next of prev_node as new_node */
        prev_Node.next = new_node;

        /* 6. Make prev_node as previous of new_node */
        new_node.prev = prev_Node;

        /* 7. Change previous of new_node's next node */
        if (new_node.next != null)
            new_node.next.prev = new_node;
    }

    // Add a node at the end of the list
    static void append(int new_data)
    {
        /* 1. allocate node
         * 2. put in the data */
        Node new_node = new Node(new_data);

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

        /* 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 == null) {
            new_node.prev = null;
            head = 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;

        /* 7. Make last node as previous of new node */
        new_node.prev = last;
    }

    // This function prints contents of
    // linked list starting from the given node
    public static void printlist(Node node)
    {
        Node last = null;
        System.out.println(
            "Traversal in forward direction");
        while (node != null) {
            System.out.print(node.data + " ");
            last = node;
            node = node.next;
        }
        System.out.println();
        System.out.println(
            "Traversal in reverse direction");
        while (last != null) {
            System.out.print(last.data + " ");
            last = last.prev;
        }
    }

    // Driver code
    public static void main(String[] args)
    {
        /* Start with the empty list */
        DLL dll = new DLL();

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

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

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

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

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

        // Insert 5, before 8.So linked
        // list becomes 1->7->5->8->6->4
        // dll.InsertBefore(dll.head.next.next, 5);

        System.out.println("Created DLL is: ");
        printlist(head);
    }
}

// This code is contributed by Sumit Ghosh

Python3

# A complete working Python3
# program to demonstrate all
# insertion methods

# A linked list node


class Node:

    # Constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = None

# Class to create a Doubly Linked List


class DoublyLinkedList:

    # Constructor for empty Doubly Linked List
    def __init__(self):
        self.head = None

    # Given a reference to the head of a list and an
    # integer, inserts a new node on the front of list
    def push(self, new_data):

        # 1. Allocates node
        # 2. Put the data in it
        new_node = Node(new_data)

        # 3. Make next of new node as head and
        # previous as None (already None)
        new_node.next = self.head

        # 4. change prev of head node to new_node
        if self.head is not None:
            self.head.prev = new_node

        # 5. move the head to point to the new node
        self.head = new_node

    # Given a node as prev_node, insert a new node after
    # the given node
    def insertAfter(self, prev_node, new_data):

        # 1. Check if the given prev_node is None
        if prev_node is None:
            print("the given previous node cannot be NULL")
            return

        # 2. allocate new node
        # 3. put in the data
        new_node = Node(new_data)

        # 4. Make net of new node as next of prev node
        new_node.next = prev_node.next

        # 5. Make prev_node as previous of new_node
        prev_node.next = new_node

        # 6. Make prev_node ass previous of new_node
        new_node.prev = prev_node

        # 7. Change previous of new_nodes's next node
        if new_node.next:
            new_node.next.prev = new_node

    # Given a reference to the head of DLL and integer,
    # appends a new node at the end
    def append(self, new_data):

        # 1. Allocates node
        # 2. Put in the data
        new_node = Node(new_data)

        # 3. This new node is going to be the last node,
        # so make next of it as None
        # (It already is initialized as None)

        # 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

        # 7. Make last node as previous of new node
        new_node.prev = last

        return

    # This function prints contents of linked list
    # starting from the given node
    def printList(self, node):

        print("\nTraversal in forward direction")
        while node:
            print("{}".format(node.data), end =" ")
            last = node
            node = node.next

        print("\nTraversal in reverse direction")
        while last:
            print("{}".format(last.data), end =" ")
            last = last.prev

# Driver code


# Start with empty list
if __name__ == "__main__":
  llist = DoublyLinkedList()

  # Insert 6. So the 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 DLL is: ", end =" ")
  llist.printList(llist.head)

# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

C#

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

// Class for Doubly Linked List
public class DLL {
    Node head; // head of list

    /* Doubly Linked list Node*/
    public class Node {
        public int data;
        public Node prev;
        public Node next;

        // Constructor to create a new node
        // next and prev is by default initialized as null
        public Node(int d) { data = d; }
    }

    // Adding a node at the front of the list
    public void push(int new_data)
    {
        /* 1. allocate node
         * 2. put in the data */
        Node new_Node = new Node(new_data);

        /* 3. Make next of new node as
        head and previous as NULL */
        new_Node.next = head;
        new_Node.prev = null;

        /* 4. change prev of head node to new node */
        if (head != null)
            head.prev = new_Node;

        /* 5. move the head to point to the new node */
        head = new_Node;
    }

    /* Given a node as prev_node, insert
    a new node after the given node */
    public void InsertAfter(Node prev_Node, int new_data)
    {

        /*1. check if the given prev_node is NULL */
        if (prev_Node == null) {
            Console.WriteLine(
                "The given previous node cannot be NULL ");
            return;
        }

        /* 2. allocate 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 the next of prev_node as new_node */
        prev_Node.next = new_node;

        /* 6. Make prev_node as previous of new_node */
        new_node.prev = prev_Node;

        /* 7. Change previous of new_node's next node */
        if (new_node.next != null)
            new_node.next.prev = new_node;
    }

    // Add a node at the end of the list
    void append(int new_data)
    {
        /* 1. allocate node
         * 2. put in the data */
        Node new_node = new Node(new_data);

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

        /* 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 == null) {
            new_node.prev = null;
            head = 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;

        /* 7. Make last node as previous of new node */
        new_node.prev = last;
    }

    // This function prints contents of
    // linked list starting from the given node
    public void printlist(Node node)
    {
        Node last = null;
        Console.WriteLine("Traversal in forward Direction");
        while (node != null) {
            Console.Write(node.data + " ");
            last = node;
            node = node.next;
        }
        Console.WriteLine();
        Console.WriteLine("Traversal in reverse direction");
        while (last != null) {
            Console.Write(last.data + " ");
            last = last.prev;
        }
    }

    /* Driver code*/
    public static void Main(String[] args)
    {
        /* Start with the empty list */
        DLL dll = new DLL();

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

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

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

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

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

        Console.WriteLine("Created DLL is: ");
        dll.printlist(dll.head);
    }
}

// This code is contributed by 29AjayKumar

Javascript

// A complete working javascript program to demonstrate all

// Class for Doubly Linked List

    var head; // head of list

    /* Doubly Linked list Node */
    class Node {

        // Constructor to create a new node
        // next and prev is by default initialized as null
constructor(d) {
            this.data = d;
            this.next = null;
            this.prev = null;
        }
    }

    // Adding a node at the front of the list
     function push(new_data) {
        /*
         * 1. allocate node 2. put in the data
         */
var new_Node = new Node(new_data);

        /* 3. Make next of new node as head and previous as NULL */
        new_Node.next = head;
        new_Node.prev = null;

        /* 4. change prev of head node to new node */
        if (head != null)
            head.prev = new_Node;

        /* 5. move the head to point to the new node */
        head = new_Node;
    }

    // Add a node before the given node
     function InsertBefore(next_node , new_data) {
        /* Check if the given nx_node is NULL */
        if (next_node == null) {
            document.write("The given next node can not be NULL");
            return;
        }

        // Allocate node, put in the data
var new_node = new Node(new_data);

        // Making prev of new node as prev of next node
        new_node.prev = next_node.prev;

        // Making prev of next node as new node
        next_node.prev = new_node;

        // Making next of new node as next node
        new_node.next = next_node;

        // Check if new node is added as head
        if (new_node.prev != null)
            new_node.prev.next = new_node;
        else
            head = new_node;
    }

    /*
     * Given a node as prev_node, insert a new node after the given node
     */
     function InsertAfter(prev_Node , new_data) {

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

        /*
         * 2. allocate 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 the next of prev_node as new_node */
        prev_Node.next = new_node;

        /* 6. Make prev_node as previous of new_node */
        new_node.prev = prev_Node;

        /* 7. Change previous of new_node's next node */
        if (new_node.next != null)
            new_node.next.prev = new_node;
    }

    // Add a node at the end of the list
    function append(new_data) {
        /*
         * 1. allocate node 2. put in the data
         */
var new_node = new Node(new_data);

var last = head; /* used in step 5 */

        /*
         * 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 == null) {
            new_node.prev = null;
            head = 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;

        /* 7. Make last node as previous of new node */
        new_node.prev = last;
    }

    // This function prints contents of
    // linked list starting from the given node
     function printlist(node) {
var last = null;
        document.write("<br/>Traversal in forward Direction<br/>");
        while (node != null) {
            document.write(node.data + " ");
            last = node;
            node = node.next;
        }
        document.write();
        document.write("<br/>Traversal in reverse direction<br/>");
        while (last != null) {
            document.write(last.data + " ");
            last = last.prev;
        }
    }

    /* Driver program to test above functions */

        /* Start with the empty list */

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

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

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

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

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

        // Insert 5, before 8.So linked
        // list becomes 1->7->5->8->6->4
        InsertBefore(head.next.next, 5);

        document.write("Created DLL is:<br/> ");
        printlist(head);

// This code is contributed by Rajput-Ji

Output

Created DLL is: 
Traversal in forward direction 
 1  7  8  6  4 
Traversal in reverse direction 
 4  6  8  7  1 

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

An alternate method by using a constructor call:

However, there is another method that uses constructor calls inside the node class in order to minimize the memory allocation work. It also minimizes the number of lines of code

Below is the implementation of the above approach:

C++

// C++ program for the above approach

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

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

    node(int value)
    { // A constructor is called here
        prev = NULL; // By default previous pointer is
                     // pointed to NULL
        data = value; // value is assigned to the data
        next = NULL; // By default next pointer is pointed
                     // to NULL
    }
};

void insert_at_head(node*& head, int value)
{

    node* n = new node(value);
    n->next = head;

    if (head != NULL) {
        head->prev = n;
    }

    head = n;
}

void insert_at_tail(node*& head, int value)
{

    if (head == NULL) {
        insert_at_head(head, value);
        return;
    }

    node* n = new node(value);
    node* temp = head;

    while (temp->next != NULL) {
        temp = temp->next;
    }
    temp->next = n;
    n->prev = temp;
}

void display(node* head)
{
    node* temp = head;
    while (temp != NULL) {
        cout << temp->data << " --> ";
        temp = temp->next;
    }
    cout << "NULL" << endl;
}

// Driver code
int main()
{
    node* head
        = NULL; // declaring an empty doubly linked list

      // Function call
    insert_at_tail(head, 1);
    insert_at_tail(head, 2);
    insert_at_tail(head, 3);
    insert_at_tail(head, 4);
    insert_at_tail(head, 5);

    cout << "After insertion at tail: ";
    display(head);

    cout << "After insertion at head: ";
    insert_at_head(head, 0);

    display(head);
    return 0;
}

Java

// Java program for the above approach
import java.io.*;

class node {
  node prev;
  int data;
  node next;
  node(int value) // A constructor is called here
  {
    prev = null; // By default previous pointer is
    // pointed to NULL
    data = value; // value is assigned to the data
    next = null; // By default next pointer is pointed
    // to NULL
  }
}

class GFG {

  static node head
    = null; // declaring an empty doubly linked list

  static void insert_at_head(int value)
  {
    node n = new node(value);
    n.next = head;

    if (head != null) {
      head.prev = n;
    }
    head = n;
  }

  static void insert_at_tail(int value)
  {
    if (head == null) {
      insert_at_head(value);
      return;
    }
    node n = new node(value);
    node temp = head;
    while (temp.next != null) {
      temp = temp.next;
    }
    temp.next = n;
    n.prev = temp;
  }

  static void display(node head)
  {
    node temp = head;
    while (temp != null) {
      System.out.print(temp.data + " --> ");
      temp = temp.next;
    }
    System.out.println("NULL");
  }

  public static void main(String[] args)
  {
    insert_at_tail(1);
    insert_at_tail(2);
    insert_at_tail(3);
    insert_at_tail(4);
    insert_at_tail(5);

    System.out.print("After insertion at tail: ");
    display(head);

    System.out.print("After insertion at head: ");
    insert_at_head(0);

    display(head);
  }
}

// This code is contributed by lokesh.

Python3

# Python code for above approach
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None
        self.prev = None

class DoublyLinkedList:
    def __init__(self):
        self.head = None

    def insert_at_head(self, value):
        n = Node(value)
        n.next = self.head
        if(self.head != None):
            self.head.prev = n
        self.head = n

    def insert_at_tail(self, value):

        n = Node(value)

        if self.head is None:
            self.head = n
            return

        last = self.head
        while (last.next):
            last = last.next

        last.next = n

    def display(self):
        temp = self.head
        while(temp):
            print(temp.data, end=" ")
            temp = temp.next

if __name__ == '__main__':
    dllist = DoublyLinkedList()
    dllist.insert_at_tail(1)
    dllist.insert_at_tail(2)
    dllist.insert_at_tail(3)
    dllist.insert_at_tail(4)
    dllist.insert_at_tail(5)
    print("After insertion at tail: ")
    dllist.display()
    dllist.insert_at_head(0)
    print("\nAfter insertion at head: ")
    dllist.display()

    # This code is contributed by ishankhandelwals.

C#

// C# program for the above approach

using System;

class node {
    public node prev;
    public int data;
    public node next;
    public node(int value) // A constructor is called here
    {
        prev = null; // By default previous pointer is
        // pointed to NULL
        data = value; // value is assigned to the data
        next = null; // By default next pointer is pointed
        // to NULL
    }
}

public class GFG {

    static node head
        = null; // declaring an empty doubly linked list

    static void insert_at_head(int value)
    {
        node n = new node(value);
        n.next = head;

        if (head != null) {
            head.prev = n;
        }
        head = n;
    }

    static void insert_at_tail(int value)
    {
        if (head == null) {
            insert_at_head(value);
            return;
        }
        node n = new node(value);
        node temp = head;
        while (temp.next != null) {
            temp = temp.next;
        }
        temp.next = n;
        n.prev = temp;
    }

    static void display(node head)
    {
        node temp = head;
        while (temp != null) {
            Console.Write(temp.data + " --> ");
            temp = temp.next;
        }
        Console.WriteLine("NULL");
    }

    static public void Main()
    {

        // Code
        insert_at_tail(1);
        insert_at_tail(2);
        insert_at_tail(3);
        insert_at_tail(4);
        insert_at_tail(5);

        Console.Write("After insertion at tail: ");
        display(head);

        Console.Write("After insertion at head: ");
        insert_at_head(0);

        display(head);
    }
}

// This code is contributed by lokeshmvs21.

Javascript

// A complete working javascript program to demonstrate all

// Class for Doubly Linked List

    var head; // head of list

    /* Doubly Linked list Node */
    class Node {

        // Constructor to create a new node
        // next and prev is by default initialized as null
constructor(d) {
            this.data = d;
            this.next = null;
            this.prev = null;
        }
    }

    // Adding a node at the front of the list
     function insert_at_head(new_data) {
        /*
         * 1. allocate node 2. put in the data
         */
var new_Node = new Node(new_data);

        /* 3. Make next of new node as head and previous as NULL */
        new_Node.next = head;
        new_Node.prev = null;

        /* 4. change prev of head node to new node */
        if (head != null)
            head.prev = new_Node;

        /* 5. move the head to point to the new node */
        head = new_Node;
    }

    // Add a node before the given node
     function InsertBefore(next_node , new_data) {
        /* Check if the given nx_node is NULL */
        if (next_node == null) {
            console.log("The given next node can not be NULL");
            return;
        }

        // Allocate node, put in the data
var new_node = new Node(new_data);

        // Making prev of new node as prev of next node
        new_node.prev = next_node.prev;

        // Making prev of next node as new node
        next_node.prev = new_node;

        // Making next of new node as next node
        new_node.next = next_node;

        // Check if new node is added as head
        if (new_node.prev != null)
            new_node.prev.next = new_node;
        else
            head = new_node;
    }

    /*
     * Given a node as prev_node, insert a new node after the given node
     */
     function InsertAfter(prev_Node , new_data) {

        /* 1. check if the given prev_node is NULL */
        if (prev_Node == null) {
            console.log("The given previous node cannot be NULL ");
            return;
        }

        /*
         * 2. allocate 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 the next of prev_node as new_node */
        prev_Node.next = new_node;

        /* 6. Make prev_node as previous of new_node */
        new_node.prev = prev_Node;

        /* 7. Change previous of new_node's next node */
        if (new_node.next != null)
            new_node.next.prev = new_node;
    }

    // Add a node at the end of the list
    function insert_at_tail(new_data) {
        /*
         * 1. allocate node 2. put in the data
         */
var new_node = new Node(new_data);

var last = head; /* used in step 5 */

        /*
         * 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 == null) {
            new_node.prev = null;
            head = 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;

        /* 7. Make last node as previous of new node */
        new_node.prev = last;
    }

   // This function prints contents of
    // linked list starting from the given node
     function display(node) {
var last = null;

        while (node != null) {
            console.log(node.data + "-->");
            last = node;
            node = node.next;
        }
       console.log("null");

    }

    /* Driver program to test above functions */

        /* Start with the empty list */

        insert_at_tail(1);
        insert_at_tail(2);
        insert_at_tail(3);
        insert_at_tail(4);
        insert_at_tail(5);
        console.log("After insertion at tail: ");
        display(head);
        console.log("After insertion at head: ");
        insert_at_head(0);
        display(head);

// This code is contributed by sourabhdalal0001

Output

After insertion at tail: 1 --> 2 --> 3 --> 4 --> 5 --> NULL
After insertion at head: 0 --> 1 --> 2 --> 3 --> 4 --> 5 --> NULL

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

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