Remove duplicates from an unsorted doubly linked list

Given an unsorted doubly linked list containing n nodes. The problem is to remove duplicate nodes from the given list.

Examples:

Method 1 (Naive Approach):

This is the simplest way where two loops are used. The outer loop is used to pick the elements one by one and the inner loop compares the picked element with the rest of the elements. 

Implementation:

C++

// C++ implementation to remove duplicates from an
// unsorted doubly linked list
#include <bits/stdc++.h>

using namespace std;

// a node of the doubly linked list
struct Node {
    int data;
    struct Node* next;
    struct Node* prev;
};

// Function to delete a node in a Doubly Linked List.
// head_ref --> pointer to head node pointer.
// del  -->  pointer to node to be deleted.
void deleteNode(struct Node** head_ref, struct Node* del)
{
    // base case
    if (*head_ref == NULL || del == NULL)
        return;

    // If node to be deleted is head node
    if (*head_ref == del)
        *head_ref = del->next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del->next != NULL)
        del->next->prev = del->prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del->prev != NULL)
        del->prev->next = del->next;

    // Finally, free the memory occupied by del
    free(del);
}

// function to remove duplicates from
// an unsorted doubly linked list
void removeDuplicates(struct Node** head_ref)
{
    // if DLL is empty or if it contains only
    // a single node
    if ((*head_ref) == NULL ||
        (*head_ref)->next == NULL)
        return;

    struct Node* ptr1, *ptr2;

    // pick elements one by one
    for (ptr1 = *head_ref; ptr1 != NULL; ptr1 = ptr1->next) {
        ptr2 = ptr1->next;

        // Compare the picked element with the
        // rest of the elements
        while (ptr2 != NULL) {

            // if duplicate, then delete it
            if (ptr1->data == ptr2->data) {

                // store pointer to the node next to 'ptr2'
                struct Node* next = ptr2->next;

                // delete node pointed to by 'ptr2'
                deleteNode(head_ref, ptr2);

                // update 'ptr2'
                ptr2 = next;
            }

            // else simply move to the next node
            else
                ptr2 = ptr2->next;
        }
    }
}

// Function to insert a node at the beginning
// of the Doubly Linked List
void push(struct Node** head_ref, int new_data)
{
    // allocate node
    struct Node* new_node =
        (struct Node*)malloc(sizeof(struct Node));

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

    // since we are adding at the beginning,
    // prev is always NULL
    new_node->prev = NULL;

    // link the old list of the new node
    new_node->next = (*head_ref);

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

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

// Function to print nodes in a given doubly
// linked list
void printList(struct Node* head)
{
    // if list is empty
    if (head == NULL)
        cout << "Doubly Linked list empty";

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

// Driver program to test above
int main()
{
    struct Node* head = NULL;

    // Create the doubly linked list:
    // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
    push(&head, 12);
    push(&head, 12);
    push(&head, 10);
    push(&head, 4);
    push(&head, 8);
    push(&head, 4);
    push(&head, 6);
    push(&head, 4);
    push(&head, 4);
    push(&head, 8);

    cout << "Original Doubly linked list:\n";
    printList(head);

    /* remove duplicate nodes */
    removeDuplicates(&head);

    cout << "\nDoubly linked list after "
            "removing duplicates:\n";
    printList(head);

    return 0;
}

Java

// Java implementation to remove duplicates
// from an unsorted doubly linked list
import java.util.*;
import java.io.*;

public class GFG
{

// a node of the doubly linked list
static class Node
{
    int data;
    Node next;
    Node prev;
}

// Function to delete a node in a Doubly Linked List.
// head_ref -. pointer to head node pointer.
// del -. pointer to node to be deleted.
static Node deleteNode(Node head_ref, Node del)
{
    // base case
    if (head_ref == null || del == null)
        return head_ref;

    // If node to be deleted is head node
    if (head_ref == del)
        head_ref = del.next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del.next != null)
        del.next.prev = del.prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del.prev != null)
        del.prev.next = del.next;
    return head_ref;
}

// function to remove duplicates from
// an unsorted doubly linked list
static Node removeDuplicates(Node head_ref)
{
    // if DLL is empty or if it contains only
    // a single node
    if ((head_ref) == null ||
        (head_ref).next == null)
        return head_ref;;

    Node ptr1, ptr2;

    // pick elements one by one
    for (ptr1 = head_ref;
         ptr1 != null; ptr1 = ptr1.next)
    {
        ptr2 = ptr1.next;

        // Compare the picked element with the
        // rest of the elements
        while (ptr2 != null)
        {

            // if duplicate, then delete it
            if (ptr1.data == ptr2.data)
            {

                // store pointer to the node next to 'ptr2'
                Node next = ptr2.next;

                // delete node pointed to by 'ptr2'
                head_ref = deleteNode(head_ref, ptr2);

                // update 'ptr2'
                ptr2 = next;
            }

            // else simply move to the next node
            else
                ptr2 = ptr2.next;
        }
    }
    return head_ref;
}

// Function to insert a node at the beginning
// of the Doubly Linked List
static Node push(Node head_ref, int new_data)
{
    // allocate node
    Node new_node = new Node();

    // put in the data
    new_node.data = new_data;

    // since we are adding at the beginning,
    // prev is always null
    new_node.prev = null;

    // link the old list of the new node
    new_node.next = (head_ref);

    // change prev of head node to new node
    if ((head_ref) != null)
        (head_ref).prev = new_node;

    // move the head to point to the new node
    (head_ref) = new_node;
    return head_ref;
}

// Function to print nodes in a
// given doubly linked list
static void printList( Node head)
{
    // if list is empty
    if (head == null)
        System.out.print("Doubly Linked list empty");

    while (head != null)
    {
        System.out.print( head.data + " ");
        head = head.next;
    }
}

// Driver Code
public static void main(String args[])
{
    Node head = null;

    // Create the doubly linked list:
    // 8<.4<.4<.6<.4<.8<.4<.10<.12<.12
    head = push(head, 12);
    head = push(head, 12);
    head = push(head, 10);
    head = push(head, 4);
    head = push(head, 8);
    head = push(head, 4);
    head = push(head, 6);
    head = push(head, 4);
    head = push(head, 4);
    head = push(head, 8);

    System.out.print("Original Doubly linked list:\n");
    printList(head);

    /* remove duplicate nodes */
    head=removeDuplicates(head);

    System.out.print("\nDoubly linked list after" +
                        " removing duplicates:\n");
    printList(head);
}
}

// This code is contributed by Arnab Kundu

Python

# Python implementation to remove duplicates
# from an unsorted doubly linked list

# Node of a linked list
class Node:
    def __init__(self, data = None, next = None):
        self.next = next
        self.data = data

# Function to delete a node in a Doubly Linked List.
# head_ref -. pointer to head node pointer.
# del -. pointer to node to be deleted.
def deleteNode(head_ref,del_):

    # base case
    if (head_ref == None or del_ == None):
        return head_ref

    # If node to be deleted is head node
    if (head_ref == del_):
        head_ref = del_.next

    # Change next only if node to be deleted
    # is NOT the last node
    if (del_.next != None):
        del_.next.prev = del_.prev

    # Change prev only if node to be deleted
    # is NOT the first node
    if (del_.prev != None):
        del_.prev.next = del_.next
    return head_ref

# function to remove duplicates from
# an unsorted doubly linked list
def removeDuplicates( head_ref):

    # if DLL is empty or if it contains only
    # a single node
    if ((head_ref) == None or (head_ref).next == None):
        return head_ref

    ptr1 = head_ref
    ptr2 = None

    # pick elements one by one
    while(ptr1 != None) :
        ptr2 = ptr1.next

        # Compare the picked element with the
        # rest of the elements
        while (ptr2 != None):

            # if duplicate, then delete it
            if (ptr1.data == ptr2.data):

                # store pointer to the node next to 'ptr2'
                next = ptr2.next

                # delete node pointed to by 'ptr2'
                head_ref = deleteNode(head_ref, ptr2)

                # update 'ptr2'
                ptr2 = next

            # else simply move to the next node
            else:
                ptr2 = ptr2.next
        ptr1 = ptr1.next
    return head_ref

# Function to insert a node at the beginning
# of the Doubly Linked List
def push( head_ref, new_data):

    # allocate node
    new_node = Node()

    # put in the data
    new_node.data = new_data

    # since we are adding at the beginning,
    # prev is always None
    new_node.prev = None

    # link the old list of the new node
    new_node.next = (head_ref)

    # change prev of head node to new node
    if ((head_ref) != None):
        (head_ref).prev = new_node

    # move the head to point to the new node
    (head_ref) = new_node
    return head_ref

# Function to print nodes in a
# given doubly linked list
def printList( head):

    # if list is empty
    if (head == None):
        print("Doubly Linked list empty")

    while (head != None):
        print( head.data ,end= " ")
        head = head.next

# Driver Code
head = None

# Create the doubly linked list:
# 8<.4<.4<.6<.4<.8<.4<.10<.12<.12
head = push(head, 12)
head = push(head, 12)
head = push(head, 10)
head = push(head, 4)
head = push(head, 8)
head = push(head, 4)
head = push(head, 6)
head = push(head, 4)
head = push(head, 4)
head = push(head, 8)

print("Original Doubly linked list:")
printList(head)

# remove duplicate nodes */
head=removeDuplicates(head)

print("\nDoubly linked list after removing duplicates:")
printList(head)

# This code is contributed by Arnab Kundu

C#

// C# implementation to remove duplicates
// from an unsorted doubly linked list
using System;

class GFG
{

// a node of the doubly linked list
public class Node
{
    public int data;
    public Node next;
    public Node prev;
}

// Function to delete a node in a Doubly Linked List.
// head_ref -. pointer to head node pointer.
// del -. pointer to node to be deleted.
static Node deleteNode(Node head_ref, Node del)
{
    // base case
    if (head_ref == null || del == null)
        return head_ref;

    // If node to be deleted is head node
    if (head_ref == del)
        head_ref = del.next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del.next != null)
        del.next.prev = del.prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del.prev != null)
        del.prev.next = del.next;
    return head_ref;
}

// function to remove duplicates from
// an unsorted doubly linked list
static Node removeDuplicates(Node head_ref)
{
    // if DLL is empty or if it contains only
    // a single node
    if ((head_ref) == null ||
        (head_ref).next == null)
        return head_ref;;

    Node ptr1, ptr2;

    // pick elements one by one
    for (ptr1 = head_ref;
        ptr1 != null; ptr1 = ptr1.next)
    {
        ptr2 = ptr1.next;

        // Compare the picked element with the
        // rest of the elements
        while (ptr2 != null)
        {

            // if duplicate, then delete it
            if (ptr1.data == ptr2.data)
            {

                // store pointer to the node next to 'ptr2'
                Node next = ptr2.next;

                // delete node pointed to by 'ptr2'
                head_ref = deleteNode(head_ref, ptr2);

                // update 'ptr2'
                ptr2 = next;
            }

            // else simply move to the next node
            else
                ptr2 = ptr2.next;
        }
    }
    return head_ref;
}

// Function to insert a node at the beginning
// of the Doubly Linked List
static Node push(Node head_ref, int new_data)
{
    // allocate node
    Node new_node = new Node();

    // put in the data
    new_node.data = new_data;

    // since we are adding at the beginning,
    // prev is always null
    new_node.prev = null;

    // link the old list of the new node
    new_node.next = (head_ref);

    // change prev of head node to new node
    if ((head_ref) != null)
        (head_ref).prev = new_node;

    // move the head to point to the new node
    (head_ref) = new_node;
    return head_ref;
}

// Function to print nodes in a
// given doubly linked list
static void printList( Node head)
{
    // if list is empty
    if (head == null)
        Console.Write("Doubly Linked list empty");

    while (head != null)
    {
        Console.Write( head.data + " ");
        head = head.next;
    }
}

// Driver Code
public static void Main(String []args)
{
    Node head = null;

    // Create the doubly linked list:
    // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
    head = push(head, 12);
    head = push(head, 12);
    head = push(head, 10);
    head = push(head, 4);
    head = push(head, 8);
    head = push(head, 4);
    head = push(head, 6);
    head = push(head, 4);
    head = push(head, 4);
    head = push(head, 8);

    Console.Write("Original Doubly linked list:\n");
    printList(head);

    /* remove duplicate nodes */
    head=removeDuplicates(head);

    Console.Write("\nDoubly linked list after" +
                     " removing duplicates:\n");
    printList(head);
}
}

// This code is contributed by PrinciRaj1992

Javascript

<script>
// javascript implementation to remove duplicates
// from an unsorted doubly linked list    // a node of the doubly linked list
     class Node {
        constructor() {
            this.data = 0;
            this.prev = null;
            this.next = null;
        }
    }
    // Function to delete a node in a Doubly Linked List.
    // head_ref -. pointer to head node pointer.
    // del -. pointer to node to be deleted.
    function deleteNode(head_ref,  del) {
        // base case
        if (head_ref == null || del == null)
            return head_ref;

        // If node to be deleted is head node
        if (head_ref == del)
            head_ref = del.next;

        // Change next only if node to be deleted
        // is NOT the last node
        if (del.next != null)
            del.next.prev = del.prev;

        // Change prev only if node to be deleted
        // is NOT the first node
        if (del.prev != null)
            del.prev.next = del.next;
        return head_ref;
    }

    // function to remove duplicates from
    // an unsorted doubly linked list
    function removeDuplicates(head_ref) {
        // if DLL is empty or if it contains only
        // a single node
        if ((head_ref) == null || (head_ref).next == null)
            return head_ref;


        var ptr1, ptr2;

        // pick elements one by one
        for (ptr1 = head_ref; ptr1 != null; ptr1 = ptr1.next) {
            ptr2 = ptr1.next;

            // Compare the picked element with the
            // rest of the elements
            while (ptr2 != null) {

                // if duplicate, then delete it
                if (ptr1.data == ptr2.data) {

                    // store pointer to the node next to 'ptr2'
                    var next = ptr2.next;

                    // delete node pointed to by 'ptr2'
                    head_ref = deleteNode(head_ref, ptr2);

                    // update 'ptr2'
                    ptr2 = next;
                }

                // else simply move to the next node
                else
                    ptr2 = ptr2.next;
            }
        }
        return head_ref;
    }

    // Function to insert a node at the beginning
    // of the Doubly Linked List
    function push(head_ref , new_data) {
        // allocate node
        var new_node = new Node();

        // put in the data
        new_node.data = new_data;

        // since we are adding at the beginning,
        // prev is always null
        new_node.prev = null;

        // link the old list of the new node
        new_node.next = (head_ref);

        // change prev of head node to new node
        if ((head_ref) != null)
            (head_ref).prev = new_node;

        // move the head to point to the new node
        (head_ref) = new_node;
         return head_ref;
    }

    // Function to print nodes in a
    // given doubly linked list
    function printList(head) {
        // if list is empty
        if (head == null)
            document.write("Doubly Linked list empty");

        while (head != null) {
            document.write(head.data + " ");
            head = head.next;
        }
    }

    // Driver Code

        var head = null;

        // Create the doubly linked list:
        // 8<.4<.4<.6<.4<.8<.4<.10<.12<.12
        head = push(head, 12);
        head = push(head, 12);
        head = push(head, 10);
        head = push(head, 4);
        head = push(head, 8);
        head = push(head, 4);
        head = push(head, 6);
        head = push(head, 4);
        head = push(head, 4);
        head = push(head, 8);

        document.write("Original Doubly linked list:<br/>");
        printList(head);

        /* remove duplicate nodes */
        head = removeDuplicates(head);

        document.write("<br/>Doubly linked list after"
        + " removing duplicates:<br/>");
        printList(head);

// This code contributed by gauravrajput1
</script>

Output

Original Doubly linked list:n8 4 4 6 4 8 4 10 12 12 
Doubly linked list after removing duplicates:n8 4 6 10 12 

Time Complexity: O(n²) 
Auxiliary Space: O(1)

Method 2 (Sorting): Following are the steps: 

1. Sort the elements of the doubly linked list using Merge Sort. Refer this post.
2. Remove duplicates in linear time using the algorithm to remove duplicates from a sorted doubly linked list.

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

Note that this method doesn’t preserve the original order of elements.

Method 3 Efficient Approach(Hashing):

We traverse the doubly linked list from head to end. For every newly encountered element, we check whether it is in the hash table: if yes, we remove it; otherwise we put it in the hash table. Hash table is implemented using unordered_set in C++. 

Implementation:

C++

// C++ implementation to remove duplicates from an
// unsorted doubly linked list
#include <bits/stdc++.h>

using namespace std;

// a node of the doubly linked list
struct Node {
    int data;
    struct Node* next;
    struct Node* prev;
};

// Function to delete a node in a Doubly Linked List.
// head_ref --> pointer to head node pointer.
// del  -->  pointer to node to be deleted.
void deleteNode(struct Node** head_ref, struct Node* del)
{
    // base case
    if (*head_ref == NULL || del == NULL)
        return;

    // If node to be deleted is head node
    if (*head_ref == del)
        *head_ref = del->next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del->next != NULL)
        del->next->prev = del->prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del->prev != NULL)
        del->prev->next = del->next;

    // Finally, free the memory occupied by del
    free(del);
}

// function to remove duplicates from
// an unsorted doubly linked list
void removeDuplicates(struct Node** head_ref)
{
    // if doubly linked list is empty
    if ((*head_ref) == NULL)
        return;

    // unordered_set 'us' implemented as hash table
    unordered_set<int> us;

    struct Node* current = *head_ref, *next;

    // traverse up to the end of the list
    while (current != NULL) {

        // if current data is seen before
        if (us.find(current->data) != us.end()) {

            // store pointer to the node next to
            // 'current' node
            next = current->next;

            // delete the node pointed to by 'current'
            deleteNode(head_ref, current);

            // update 'current'
            current = next;
        }

        else {

            // insert the current data in 'us'
            us.insert(current->data);

            // move to the next node
            current = current->next;
        }
    }
}

// Function to insert a node at the beginning
// of the Doubly Linked List
void push(struct Node** head_ref, int new_data)
{
    // allocate node
    struct Node* new_node =
         (struct Node*)malloc(sizeof(struct Node));

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

    // since we are adding at the beginning,
    // prev is always NULL
    new_node->prev = NULL;

    // link the old list of the new node
    new_node->next = (*head_ref);

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

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

// Function to print nodes in a given doubly
// linked list
void printList(struct Node* head)
{
    // if list is empty
    if (head == NULL)
        cout << "Doubly Linked list empty";

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

// Driver program to test above
int main()
{
    struct Node* head = NULL;

    // Create the doubly linked list:
    // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
    push(&head, 12);
    push(&head, 12);
    push(&head, 10);
    push(&head, 4);
    push(&head, 8);
    push(&head, 4);
    push(&head, 6);
    push(&head, 4);
    push(&head, 4);
    push(&head, 8);

    cout << "Original Doubly linked list:n";
    printList(head);

    /* remove duplicate nodes */
    removeDuplicates(&head);

    cout << "\nDoubly linked list after "
             "removing duplicates:n";
    printList(head);

    return 0;
}

Java

// Java implementation to remove duplicates
// from an unsorted doubly linked list
import java.util.*;

public class GFG
{

// a node of the doubly linked list
static class Node
{
    int data;
    Node next;
    Node prev;
};

// Function to delete a node in a Doubly Linked List.
// head_ref --> pointer to head node pointer.
// del --> pointer to node to be deleted.
static Node deleteNode(Node head_ref, Node del)
{
    // base case
    if (head_ref == null || del == null)
        return null;

    // If node to be deleted is head node
    if (head_ref == del)
        head_ref = del.next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del.next != null)
        del.next.prev = del.prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del.prev != null)
        del.prev.next = del.next;

    return head_ref;
}

// function to remove duplicates from
// an unsorted doubly linked list
static Node removeDuplicates(Node head_ref)
{
    // if doubly linked list is empty
    if ((head_ref) == null)
        return null;

    // unordered_set 'us' implemented as hash table
    HashSet<Integer> us = new HashSet<>();

    Node current = head_ref, next;

    // traverse up to the end of the list
    while (current != null)
    {

        // if current data is seen before
        if (us.contains(current.data))
        {

            // store pointer to the node next to
            // 'current' node
            next = current.next;

            // delete the node pointed to by 'current'
            head_ref = deleteNode(head_ref, current);

            // update 'current'
            current = next;
        }
        else
        {

            // insert the current data in 'us'
            us.add(current.data);

            // move to the next node
            current = current.next;
        }
    }
    return head_ref;
}

// Function to insert a node at the
// beginning of the Doubly Linked List
static Node push(Node head_ref,
                 int new_data)
{
    // allocate node
    Node new_node = new Node();

    // put in the data
    new_node.data = new_data;

    // since we are adding at the beginning,
    // prev is always null
    new_node.prev = null;

    // link the old list of the new node
    new_node.next = (head_ref);

    // change prev of head node to new node
    if ((head_ref) != null)
        (head_ref).prev = new_node;

    // move the head to point to the new node
    (head_ref) = new_node;
    return head_ref;
}

// Function to print nodes in a given doubly
// linked list
static void printList(Node head)
{
    // if list is empty
    if (head == null)
        System.out.print("Doubly Linked list empty");

    while (head != null)
    {
        System.out.print(head.data + " ");
        head = head.next;
    }
}

// Driver Code
public static void main(String[] args)
{
    Node head = null;

    // Create the doubly linked list:
    // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
    head = push(head, 12);
    head = push(head, 12);
    head = push(head, 10);
    head = push(head, 4);
    head = push(head, 8);
    head = push(head, 4);
    head = push(head, 6);
    head = push(head, 4);
    head = push(head, 4);
    head = push(head, 8);

    System.out.println("Original Doubly linked list:");
    printList(head);

    /* remove duplicate nodes */
    head = removeDuplicates(head);

    System.out.println("\nDoubly linked list after " +
                              "removing duplicates:");
    printList(head);
}
}

// This code is contributed by Rajput-Ji

Python3

# Python3 implementation to remove duplicates
# from an unsorted doubly linked list

# a node of the doubly linked list
class Node:
    def __init__(self):
        self.data = 0
        self.next = None
        self.prev = None

# Function to delete a node in a Doubly Linked List.
# head_ref --> pointer to head node pointer.
# del --> pointer to node to be deleted.
def deleteNode( head_ref, del_):

    # base case
    if (head_ref == None or del_ == None):
        return None

    # If node to be deleted is head node
    if (head_ref == del_):
        head_ref = del_.next

    # Change next only if node to be deleted
    # is NOT the last node
    if (del_.next != None):
        del_.next.prev = del_.prev

    # Change prev only if node to be deleted
    # is NOT the first node
    if (del_.prev != None):
        del_.prev.next = del_.next

    return head_ref

# function to remove duplicates from
# an unsorted doubly linked list
def removeDuplicates(head_ref):

    # if doubly linked list is empty
    if ((head_ref) == None):
        return None

    # unordered_set 'us' implemented as hash table
    us = set()

    current = head_ref
    next = None

    # traverse up to the end of the list
    while (current != None):

        # if current data is seen before
        if ((current.data) in us):

            # store pointer to the node next to
            # 'current' node
            next = current.next

            # delete the node pointed to by 'current'
            head_ref = deleteNode(head_ref, current)

            # update 'current'
            current = next

        else:

            # insert the current data in 'us'
            us.add(current.data)

            # move to the next node
            current = current.next

    return head_ref

# Function to insert a node at the
# beginning of the Doubly Linked List
def push(head_ref,new_data):

    # allocate node
    new_node = Node()

    # put in the data
    new_node.data = new_data

    # since we are adding at the beginning,
    # prev is always None
    new_node.prev = None

    # link the old list of the new node
    new_node.next = (head_ref)

    # change prev of head node to new node
    if ((head_ref) != None):
        (head_ref).prev = new_node

    # move the head to point to the new node
    (head_ref) = new_node
    return head_ref

# Function to print nodes in a given doubly
# linked list
def printList( head):

    # if list is empty
    if (head == None):
        print("Doubly Linked list empty")

    while (head != None):

        print(head.data , end=" ")
        head = head.next

# Driver Code

head = None

# Create the doubly linked list:
# 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
head = push(head, 12)
head = push(head, 12)
head = push(head, 10)
head = push(head, 4)
head = push(head, 8)
head = push(head, 4)
head = push(head, 6)
head = push(head, 4)
head = push(head, 4)
head = push(head, 8)

print("Original Doubly linked list:")
printList(head)

# remove duplicate nodes
head = removeDuplicates(head)

print("\nDoubly linked list after removing duplicates:")
printList(head)

# This code is contributed by Arnab Kundu

C#

// C# implementation to remove duplicates
// from an unsorted doubly linked list
using System;
using System.Collections.Generic;

class GFG
{

// a node of the doubly linked list
public class Node
{
    public int data;
    public Node next;
    public Node prev;
};

// Function to delete a node in a Doubly Linked List.
// head_ref --> pointer to head node pointer.
// del --> pointer to node to be deleted.
static Node deleteNode(Node head_ref, Node del)
{
    // base case
    if (head_ref == null || del == null)
        return null;

    // If node to be deleted is head node
    if (head_ref == del)
        head_ref = del.next;

    // Change next only if node to be deleted
    // is NOT the last node
    if (del.next != null)
        del.next.prev = del.prev;

    // Change prev only if node to be deleted
    // is NOT the first node
    if (del.prev != null)
        del.prev.next = del.next;

    return head_ref;
}

// function to remove duplicates from
// an unsorted doubly linked list
static Node removeDuplicates(Node head_ref)
{
    // if doubly linked list is empty
    if ((head_ref) == null)
        return null;

    // unordered_set 'us' implemented as hash table
    HashSet<int> us = new HashSet<int>();

    Node current = head_ref, next;

    // traverse up to the end of the list
    while (current != null)
    {

        // if current data is seen before
        if (us.Contains(current.data))
        {

            // store pointer to the node next to
            // 'current' node
            next = current.next;

            // delete the node pointed to by 'current'
            head_ref = deleteNode(head_ref, current);

            // update 'current'
            current = next;
        }
        else
        {

            // insert the current data in 'us'
            us.Add(current.data);

            // move to the next node
            current = current.next;
        }
    }
    return head_ref;
}

// Function to insert a node at the
// beginning of the Doubly Linked List
static Node push(Node head_ref,
                 int new_data)
{
    // allocate node
    Node new_node = new Node();

    // put in the data
    new_node.data = new_data;

    // since we are adding at the beginning,
    // prev is always null
    new_node.prev = null;

    // link the old list of the new node
    new_node.next = (head_ref);

    // change prev of head node to new node
    if ((head_ref) != null)
        (head_ref).prev = new_node;

    // move the head to point to the new node
    (head_ref) = new_node;
    return head_ref;
}

// Function to print nodes in a
// given doubly linked list
static void printList(Node head)
{
    // if list is empty
    if (head == null)
        Console.Write("Doubly Linked list empty");

    while (head != null)
    {
        Console.Write(head.data + " ");
        head = head.next;
    }
}

// Driver Code
public static void Main(String[] args)
{
    Node head = null;

    // Create the doubly linked list:
    // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
    head = push(head, 12);
    head = push(head, 12);
    head = push(head, 10);
    head = push(head, 4);
    head = push(head, 8);
    head = push(head, 4);
    head = push(head, 6);
    head = push(head, 4);
    head = push(head, 4);
    head = push(head, 8);

    Console.WriteLine("Original Doubly linked list:");
    printList(head);

    /* remove duplicate nodes */
    head = removeDuplicates(head);

    Console.WriteLine("\nDoubly linked list after " +
                             "removing duplicates:");
    printList(head);
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>
// javascript implementation to remove duplicates
// from an unsorted doubly linked list

    // a node of the doubly linked list
class Node {
    constructor(val) {
        this.data = val;
        this.prev = null;
        this.next = null;
    }
}
    // Function to delete a node in a Doubly Linked List.
    // head_ref --> pointer to head node pointer.
    // del --> pointer to node to be deleted.
    function deleteNode(head_ref,  del) {
        // base case
        if (head_ref == null || del == null)
            return null;

        // If node to be deleted is head node
        if (head_ref == del)
            head_ref = del.next;

        // Change next only if node to be deleted
        // is NOT the last node
        if (del.next != null)
            del.next.prev = del.prev;

        // Change prev only if node to be deleted
        // is NOT the first node
        if (del.prev != null)
            del.prev.next = del.next;

        return head_ref;
    }

    // function to remove duplicates from
    // an unsorted doubly linked list
    function removeDuplicates(head_ref) {
        // if doubly linked list is empty
        if ((head_ref) == null)
            return null;

        // unordered_set 'us' implemented as hash table
        var us = new Set();

var current = head_ref, next;

        // traverse up to the end of the list
        while (current != null) {

            // if current data is seen before
            if (us.has(current.data)) {

                // store pointer to the node next to
                // 'current' node
                next = current.next;

                // delete the node pointed to by 'current'
                head_ref = deleteNode(head_ref, current);

                // update 'current'
                current = next;
            } else {

                // insert the current data in 'us'
                us.add(current.data);

                // move to the next node
                current = current.next;
            }
        }
        return head_ref;
    }

    // Function to insert a node at the
    // beginning of the Doubly Linked List
    function push(head_ref , new_data) {
        // allocate node
var new_node = new Node();

        // put in the data
        new_node.data = new_data;

        // since we are adding at the beginning,
        // prev is always null
        new_node.prev = null;

        // link the old list of the new node
        new_node.next = (head_ref);

        // change prev of head node to new node
        if ((head_ref) != null)
            (head_ref).prev = new_node;

        // move the head to point to the new node
        (head_ref) = new_node;
        return head_ref;
    }

    // Function to print nodes in a given doubly
    // linked list
    function printList(head) {
        // if list is empty
        if (head == null)
            document.write("Doubly Linked list empty");

        while (head != null) {
            document.write(head.data + " ");
            head = head.next;
        }
    }

    // Driver Code

var head = null;

        // Create the doubly linked list:
        // 8<->4<->4<->6<->4<->8<->4<->10<->12<->12
        head = push(head, 12);
        head = push(head, 12);
        head = push(head, 10);
        head = push(head, 4);
        head = push(head, 8);
        head = push(head, 4);
        head = push(head, 6);
        head = push(head, 4);
        head = push(head, 4);
        head = push(head, 8);

        document.write("Original Doubly linked list:<br/>");
        printList(head);

        /* remove duplicate nodes */
        head = removeDuplicates(head);

        document.write("<br/>Doubly linked list after " + "removing duplicates:<br/>");
        printList(head);

// This code contributed by gauravrajput1
</script>

Output

Original Doubly linked list:n8 4 4 6 4 8 4 10 12 12 
Doubly linked list after removing duplicates:n8 4 6 10 12 

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

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.