# 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:

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Method 1 (Naive Approach):
This is the simplest way where two loops are used. Outer loop is used to pick the elements one by one and inner loop compares the picked element with rest of the elements.

```// C++ implementation to remove duplicates from an
#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.
// 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

// 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
{
// if DLL is empty or if it contains only
// a single node
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'

// 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 begining,
// prev is always NULL
new_node->prev = NULL;

// link the old list off the new node

// change prev of head node to new node

// move the head to point to the new node
}

// Function to print nodes in a given doubly
{
// if list is empty
cout << "Doubly Linked list empty";

cout << head->data << " ";
}
}

// Driver program to test above
int main()
{

// Create the doubly linked list:
// 8<->4<->4<->6<->4<->8<->4<->10<->12<->12

cout << "Original Doubly linked list:n";

/* remove duplicate nodes */

cout << "\nDoubly linked list after "
"removing duplicates:n";

return 0;
}
```

Output:

```Original Doubly linked list:
8 4 4 6 4 8 4 10 12 12
Doubly linked list after removing duplicates:
8 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++.

```// C++ implementation to remove duplicates from an
#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.
// 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

// 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
{
// if doubly linked list is empty
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'

// 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 off the new node

// change prev of head node to new node

// move the head to point to the new node
}

// Function to print nodes in a given doubly
{
// if list is empty
cout << "Doubly Linked list empty";

cout << head->data << " ";
}
}

// Driver program to test above
int main()
{

// Create the doubly linked list:
// 8<->4<->4<->6<->4<->8<->4<->10<->12<->12

cout << "Original Doubly linked list:n";

/* remove duplicate nodes */

cout << "\nDoubly linked list after "
"removing duplicates:n";

return 0;
}
```

Output:

```Original Doubly linked list:
8 4 4 6 4 8 4 10 12 12
Doubly linked list after removing duplicates:
8 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 contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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