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# Remove duplicates from an unsorted linked list

• Difficulty Level : Easy
• Last Updated : 20 Sep, 2022

Given an unsorted list of nodes. The task is to remove duplicates from the list.

Examples:

Output: 12->11->21->41->43.
Explanation: Second occurrence o 12 and 21 is removed

Output: 12->11->21->41->43.

Naive Approach:

Use two loops, 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.

Below is the Implementation of the above approach:

## C++

 `/* C++ Program to remove duplicates in an unsorted``   ``linked list */``#include ``using` `namespace` `std;` `/* A linked list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `// Utility function to create a new Node``struct` `Node* newNode(``int` `data)``{``    ``Node* temp = ``new` `Node;``    ``temp->data = data;``    ``temp->next = NULL;``    ``return` `temp;``}` `/* Function to remove duplicates from a``   ``unsorted linked list */``void` `removeDuplicates(``struct` `Node* start)``{``    ``struct` `Node *ptr1, *ptr2, *dup;``    ``ptr1 = start;` `    ``/* Pick elements one by one */``    ``while` `(ptr1 != NULL && ptr1->next != NULL) {``        ``ptr2 = ptr1;` `        ``/* Compare the picked element with rest``           ``of the elements */``        ``while` `(ptr2->next != NULL) {``            ``/* If duplicate then delete it */``            ``if` `(ptr1->data == ptr2->next->data) {``                ``/* sequence of steps is important here */``                ``dup = ptr2->next;``                ``ptr2->next = ptr2->next->next;``                ``delete` `(dup);``            ``}``            ``else` `/* This is tricky */``                ``ptr2 = ptr2->next;``        ``}``        ``ptr1 = ptr1->next;``    ``}``}` `/* Function to print nodes in a given linked list */``void` `printList(``struct` `Node* node)``{``    ``while` `(node != NULL) {``        ``printf``(``"%d "``, node->data);``        ``node = node->next;``    ``}``}` `// Driver code``int` `main()``{``    ``/* The constructed linked list is:``     ``10->12->11->11->12->11->10*/``    ``struct` `Node* start = newNode(10);``    ``start->next = newNode(12);``    ``start->next->next = newNode(11);``    ``start->next->next->next = newNode(11);``    ``start->next->next->next->next = newNode(12);``    ``start->next->next->next->next->next = newNode(11);``    ``start->next->next->next->next->next->next = newNode(10);` `    ``printf``(``"Linked list before removing duplicates "``);``    ``printList(start);` `    ``removeDuplicates(start);` `    ``printf``(``"\nLinked list after removing duplicates "``);``    ``printList(start);``    ``return` `0;``}`

## C

 `/* C Program to remove duplicates in an unsorted``   ``linked list */``#include``#include` `/* A linked list node */``typedef` `struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``} Node;` `// Utility function to create a new Node``Node* newNode(``int` `data)``{``    ``Node* temp = (Node*)``malloc``(``sizeof``(Node));``    ``temp->data = data;``    ``temp->next = NULL;``    ``return` `temp;``}` `/* Function to remove duplicates from a``   ``unsorted linked list */``void` `removeDuplicates(Node* start)``{``    ``Node *ptr1, *ptr2, *dup;``    ``ptr1 = start;` `    ``/* Pick elements one by one */``    ``while` `(ptr1 != NULL && ptr1->next != NULL) {``        ``ptr2 = ptr1;` `        ``/* Compare the picked element with rest``           ``of the elements */``        ``while` `(ptr2->next != NULL) {``            ``/* If duplicate then delete it */``            ``if` `(ptr1->data == ptr2->next->data) {``                ``/* sequence of steps is important here */``                ``dup = ptr2->next;``                ``ptr2->next = ptr2->next->next;``                ``free``(dup);``            ``}``            ``else` `/* This is tricky */``                ``ptr2 = ptr2->next;``        ``}``        ``ptr1 = ptr1->next;``    ``}``}` `/* Function to print nodes in a given linked list */``void` `printList(``struct` `Node* node)``{``    ``while` `(node != NULL) {``        ``printf``(``"%d "``, node->data);``        ``node = node->next;``    ``}``}` `/* Driver program to test above function */``int` `main()``{``    ``/* The constructed linked list is:``     ``10->12->11->11->12->11->10*/``    ``struct` `Node* start = newNode(10);``    ``start->next = newNode(12);``    ``start->next->next = newNode(11);``    ``start->next->next->next = newNode(11);``    ``start->next->next->next->next = newNode(12);``    ``start->next->next->next->next->next = newNode(11);``    ``start->next->next->next->next->next->next = newNode(10);` `    ``printf``(``"Linked list before removing duplicates "``);``    ``printList(start);` `    ``removeDuplicates(start);` `    ``printf``(``"\nLinked list after removing duplicates "``);``    ``printList(start);` `    ``return` `0;``}` `// This code is contributed by Aditya Kumar (adityakumar129)`

## Java

 `// Java program to remove duplicates from unsorted``// linked list` `class` `LinkedList {` `    ``static` `Node head;` `    ``static` `class` `Node {` `        ``int` `data;``        ``Node next;` `        ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Function to remove duplicates from an``       ``unsorted linked list */``    ``void` `remove_duplicates()``    ``{``        ``Node ptr1 = ``null``, ptr2 = ``null``, dup = ``null``;``        ``ptr1 = head;` `        ``/* Pick elements one by one */``        ``while` `(ptr1 != ``null` `&& ptr1.next != ``null``) {``            ``ptr2 = ptr1;` `            ``/* Compare the picked element with rest``                ``of the elements */``            ``while` `(ptr2.next != ``null``) {` `                ``/* If duplicate then delete it */``                ``if` `(ptr1.data == ptr2.next.data) {` `                    ``/* sequence of steps is important here``                     ``*/``                    ``ptr2.next = ptr2.next.next;``                    ``System.gc();``                ``}``                ``else` `/* This is tricky */` `{``                    ``ptr2 = ptr2.next;``                ``}``            ``}``            ``ptr1 = ptr1.next;``        ``}``    ``}` `    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``System.out.print(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``public` `static` `void` `main(String[] args)``    ``{``        ``LinkedList list = ``new` `LinkedList();``        ``list.head = ``new` `Node(``10``);``        ``list.head.next = ``new` `Node(``12``);``        ``list.head.next.next = ``new` `Node(``11``);``        ``list.head.next.next.next = ``new` `Node(``11``);``        ``list.head.next.next.next.next = ``new` `Node(``12``);``        ``list.head.next.next.next.next.next = ``new` `Node(``11``);``        ``list.head.next.next.next.next.next.next``            ``= ``new` `Node(``10``);` `        ``System.out.println(``            ``"Linked List before removing duplicates : "``);``        ``list.printList(head);` `        ``list.remove_duplicates();``        ``System.out.println(``"\n"``);``        ``System.out.println(``            ``"Linked List after removing duplicates : "``);``        ``list.printList(head);``    ``}``}``// This code has been contributed by Mayank Jaiswal`

## Python3

 `# Python3 program to remove duplicates``# from unsorted linked list`  `class` `Node():` `    ``def` `__init__(``self``, data):` `        ``self``.data ``=` `data``        ``self``.``next` `=` `None`  `class` `LinkedList():` `    ``def` `__init__(``self``):` `        ``# Head of list``        ``self``.head ``=` `None` `    ``def` `remove_duplicates(``self``):` `        ``ptr1 ``=` `None``        ``ptr2 ``=` `None``        ``dup ``=` `None``        ``ptr1 ``=` `self``.head` `        ``# Pick elements one by one``        ``while` `(ptr1 !``=` `None` `and` `ptr1.``next` `!``=` `None``):` `            ``ptr2 ``=` `ptr1` `            ``# Compare the picked element with rest``            ``# of the elements``            ``while` `(ptr2.``next` `!``=` `None``):` `                ``# If duplicate then delete it``                ``if` `(ptr1.data ``=``=` `ptr2.``next``.data):` `                    ``# Sequence of steps is important here``                    ``dup ``=` `ptr2.``next``                    ``ptr2.``next` `=` `ptr2.``next``.``next``                ``else``:``                    ``ptr2 ``=` `ptr2.``next` `            ``ptr1 ``=` `ptr1.``next` `    ``# Function to print nodes in a``    ``# given linked list``    ``def` `printList(``self``):``        ``temp ``=` `self``.head` `        ``while``(temp !``=` `None``):``            ``print``(temp.data, end``=``" "``)``            ``temp ``=` `temp.``next` `        ``print``()`  `# Driver code``list` `=` `LinkedList()``list``.head ``=` `Node(``10``)``list``.head.``next` `=` `Node(``12``)``list``.head.``next``.``next` `=` `Node(``11``)``list``.head.``next``.``next``.``next` `=` `Node(``11``)``list``.head.``next``.``next``.``next``.``next` `=` `Node(``12``)``list``.head.``next``.``next``.``next``.``next``.``next` `=` `Node(``11``)``list``.head.``next``.``next``.``next``.``next``.``next``.``next` `=` `Node(``10``)` `print``(``"Linked List before removing duplicates :"``)``list``.printList()``list``.remove_duplicates()``print``()``print``(``"Linked List after removing duplicates :"``)``list``.printList()` `# This code is contributed by maheshwaripiyush9`

## C#

 `// C# program to remove duplicates from unsorted``// linked list``using` `System;``class` `List_ {` `    ``Node head;` `    ``class` `Node {` `        ``public` `int` `data;``        ``public` `Node next;` `        ``public` `            ``Node(``int` `d)``        ``{``            ``data = d;``            ``next = ``null``;``        ``}``    ``}` `    ``/* Function to remove duplicates from an``         ``unsorted linked list */``    ``void` `remove_duplicates()``    ``{``        ``Node ptr1 = ``null``, ptr2 = ``null``;``        ``ptr1 = head;` `        ``/* Pick elements one by one */``        ``while` `(ptr1 != ``null` `&& ptr1.next != ``null``) {``            ``ptr2 = ptr1;` `            ``/* Compare the picked element with rest``                      ``of the elements */``            ``while` `(ptr2.next != ``null``) {` `                ``/* If duplicate then delete it */``                ``if` `(ptr1.data == ptr2.next.data) {` `                    ``/* sequence of steps is important here``                     ``*/``                    ``ptr2.next = ptr2.next.next;``                ``}``                ``else` `/* This is tricky */``                ``{``                    ``ptr2 = ptr2.next;``                ``}``            ``}``            ``ptr1 = ptr1.next;``        ``}``    ``}` `    ``void` `printList(Node node)``    ``{``        ``while` `(node != ``null``) {``            ``Console.Write(node.data + ``" "``);``            ``node = node.next;``        ``}``    ``}` `    ``// Driver Code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``List_ list = ``new` `List_();``        ``list.head = ``new` `Node(10);``        ``list.head.next = ``new` `Node(12);``        ``list.head.next.next = ``new` `Node(11);``        ``list.head.next.next.next = ``new` `Node(11);``        ``list.head.next.next.next.next = ``new` `Node(12);``        ``list.head.next.next.next.next.next = ``new` `Node(11);``        ``list.head.next.next.next.next.next.next``            ``= ``new` `Node(10);` `        ``Console.WriteLine(``            ``"Linked List_ before removing duplicates: "``);``        ``list.printList(list.head);` `        ``list.remove_duplicates();``        ``Console.WriteLine(``""``);``        ``Console.WriteLine(``            ``"Linked List_ after removing duplicates: "``);``        ``list.printList(list.head);``    ``}``}` `// This code is contributed by gauravrajput1`

## Javascript

 ``

Output

```Linked list before removing duplicates 10 12 11 11 12 11 10
Linked list after removing duplicates 10 12 11 ```

Time Complexity: O(N2)
Auxiliary Space: O(1)

Thanks to Gaurav Saxena for his help in writing this code.

## Remove duplicates from an unsorted linked list using sorting:

Follow the below steps to Implement the idea:

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

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

## Remove duplicates from an unsorted linked list using Hashing:

Traverse the link list from head to end. For every newly encountered element, check whether if it is in the hash table: if yes, we remove it; otherwise put it in the hash table.

Follow the below steps to Implement the idea:

• Create a Unordered set to keep a track of the visited elements.
• If current node is already present in the Hashset. Then delete the current node.
• Else move insert the node in the Hashset and move to the next node.
• Return

Below is the implementation of the above approach:

## C++

 `/* C++ Program to remove duplicates in an unsorted``   ``linked list */``#include ``using` `namespace` `std;` `/* A linked list node */``struct` `Node {``    ``int` `data;``    ``struct` `Node* next;``};` `// Utility function to create a new Node``struct` `Node* newNode(``int` `data)``{``    ``Node* temp = ``new` `Node;``    ``temp->data = data;``    ``temp->next = NULL;``    ``return` `temp;``}` `/* Function to remove duplicates from a``   ``unsorted linked list */``void` `removeDuplicates(``struct` `Node* start)``{``    ``// Hash to store seen values``    ``unordered_set<``int``> seen;` `    ``/* Pick elements one by one */``    ``struct` `Node* curr = start;``    ``struct` `Node* prev = NULL;``    ``while` `(curr != NULL) {``        ``// If current value is seen before``        ``if` `(seen.find(curr->data) != seen.end()) {``            ``prev->next = curr->next;``            ``delete` `(curr);``        ``}``        ``else` `{``            ``seen.insert(curr->data);``            ``prev = curr;``        ``}``        ``curr = prev->next;``    ``}``}` `/* Function to print nodes in a given linked list */``void` `printList(``struct` `Node* node)``{``    ``while` `(node != NULL) {``        ``printf``(``"%d "``, node->data);``        ``node = node->next;``    ``}``}` `/* Driver program to test above function */``int` `main()``{``    ``/* The constructed linked list is:``     ``10->12->11->11->12->11->10*/``    ``struct` `Node* start = newNode(10);``    ``start->next = newNode(12);``    ``start->next->next = newNode(11);``    ``start->next->next->next = newNode(11);``    ``start->next->next->next->next = newNode(12);``    ``start->next->next->next->next->next = newNode(11);``    ``start->next->next->next->next->next->next = newNode(10);` `    ``printf``(``"Linked list before removing duplicates : \n"``);``    ``printList(start);` `    ``removeDuplicates(start);` `    ``printf``(``"\nLinked list after removing duplicates : \n"``);``    ``printList(start);` `    ``return` `0;``}`

## Java

 `// Java program to remove duplicates``// from unsorted linkedlist` `import` `java.util.HashSet;` `public` `class` `removeDuplicates {``    ``static` `class` `node {``        ``int` `val;``        ``node next;` `        ``public` `node(``int` `val) { ``this``.val = val; }``    ``}` `    ``/* Function to remove duplicates from a``       ``unsorted linked list */``    ``static` `void` `removeDuplicate(node head)``    ``{``        ``// Hash to store seen values``        ``HashSet hs = ``new` `HashSet<>();` `        ``/* Pick elements one by one */``        ``node current = head;``        ``node prev = ``null``;``        ``while` `(current != ``null``) {``            ``int` `curval = current.val;` `            ``// If current value is seen before``            ``if` `(hs.contains(curval)) {``                ``prev.next = current.next;``            ``}``            ``else` `{``                ``hs.add(curval);``                ``prev = current;``            ``}``            ``current = current.next;``        ``}``    ``}` `    ``/* Function to print nodes in a given linked list */``    ``static` `void` `printList(node head)``    ``{``        ``while` `(head != ``null``) {``            ``System.out.print(head.val + ``" "``);``            ``head = head.next;``        ``}``    ``}` `    ``public` `static` `void` `main(String[] args)``    ``{``        ``/* The constructed linked list is:``         ``10->12->11->11->12->11->10*/``        ``node start = ``new` `node(``10``);``        ``start.next = ``new` `node(``12``);``        ``start.next.next = ``new` `node(``11``);``        ``start.next.next.next = ``new` `node(``11``);``        ``start.next.next.next.next = ``new` `node(``12``);``        ``start.next.next.next.next.next = ``new` `node(``11``);``        ``start.next.next.next.next.next.next = ``new` `node(``10``);` `        ``System.out.println(``            ``"Linked list before removing duplicates :"``);``        ``printList(start);` `        ``removeDuplicate(start);` `        ``System.out.println(``            ``"\nLinked list after removing duplicates :"``);``        ``printList(start);``    ``}``}` `// This code is contributed by Rishabh Mahrsee`

## Python3

 `# Python3 program to remove duplicates``# from unsorted linkedlist`  `class` `Node:` `    ``def` `__init__(``self``, data):` `        ``self``.data ``=` `data``        ``self``.``next` `=` `None`  `class` `LinkedList:` `    ``def` `__init__(``self``):` `        ``self``.head ``=` `None` `    ``# Function to print nodes in a``    ``# given linked list``    ``def` `printlist(``self``):` `        ``temp ``=` `self``.head` `        ``while` `(temp):``            ``print``(temp.data, end``=``" "``)``            ``temp ``=` `temp.``next` `    ``# Function to remove duplicates from a``    ``# unsorted linked list``    ``def` `removeDuplicates(``self``, head):` `        ``# Base case of empty list or``        ``# list with only one element``        ``if` `self``.head ``is` `None` `or` `self``.head.``next` `is` `None``:``            ``return` `head` `        ``# Hash to store seen values``        ``hash` `=` `set``()` `        ``current ``=` `head``        ``hash``.add(``self``.head.data)` `        ``while` `current.``next` `is` `not` `None``:` `            ``if` `current.``next``.data ``in` `hash``:``                ``current.``next` `=` `current.``next``.``next``            ``else``:``                ``hash``.add(current.``next``.data)``                ``current ``=` `current.``next` `        ``return` `head`  `# Driver code``if` `__name__ ``=``=` `"__main__"``:` `    ``# Creating Empty list``    ``llist ``=` `LinkedList()``    ``llist.head ``=` `Node(``10``)``    ``second ``=` `Node(``12``)``    ``third ``=` `Node(``11``)``    ``fourth ``=` `Node(``11``)``    ``fifth ``=` `Node(``12``)``    ``sixth ``=` `Node(``11``)``    ``seventh ``=` `Node(``10``)` `    ``# Connecting second and third``    ``llist.head.``next` `=` `second``    ``second.``next` `=` `third``    ``third.``next` `=` `fourth``    ``fourth.``next` `=` `fifth``    ``fifth.``next` `=` `sixth``    ``sixth.``next` `=` `seventh` `    ``# Printing data``    ``print``(``"Linked List before removing Duplicates."``)``    ``llist.printlist()``    ``llist.removeDuplicates(llist.head)``    ``print``(``"\nLinked List after removing duplicates."``)``    ``llist.printlist()` `# This code is contributed by rajataro0`

## C#

 `// C# program to remove duplicates``// from unsorted linkedlist``using` `System;``using` `System.Collections.Generic;` `class` `removeDuplicates {``    ``class` `node {``        ``public` `int` `val;``        ``public` `node next;` `        ``public` `node(``int` `val) { ``this``.val = val; }``    ``}` `    ``// Function to remove duplicates from a``    ``// unsorted linked list``    ``static` `void` `removeDuplicate(node head)``    ``{` `        ``// Hash to store seen values``        ``HashSet<``int``> hs = ``new` `HashSet<``int``>();` `        ``// Pick elements one by one``        ``node current = head;``        ``node prev = ``null``;``        ``while` `(current != ``null``) {``            ``int` `curval = current.val;` `            ``// If current value is seen before``            ``if` `(hs.Contains(curval)) {``                ``prev.next = current.next;``            ``}``            ``else` `{``                ``hs.Add(curval);``                ``prev = current;``            ``}``            ``current = current.next;``        ``}``    ``}` `    ``// Function to print nodes in a``    ``// given linked list``    ``static` `void` `printList(node head)``    ``{``        ``while` `(head != ``null``) {``            ``Console.Write(head.val + ``" "``);``            ``head = head.next;``        ``}``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args)``    ``{` `        ``// The constructed linked list is:``        ``// 10->12->11->11->12->11->10``        ``node start = ``new` `node(10);``        ``start.next = ``new` `node(12);``        ``start.next.next = ``new` `node(11);``        ``start.next.next.next = ``new` `node(11);``        ``start.next.next.next.next = ``new` `node(12);``        ``start.next.next.next.next.next = ``new` `node(11);``        ``start.next.next.next.next.next.next = ``new` `node(10);` `        ``Console.WriteLine(``"Linked list before removing "``                          ``+ ``"duplicates :"``);``        ``printList(start);` `        ``removeDuplicate(start);` `        ``Console.WriteLine(``"\nLinked list after removing "``                          ``+ ``"duplicates :"``);``        ``printList(start);``    ``}``}` `// This code is contributed by amal kumar choubey`

## Javascript

 ``

Output

```Linked list before removing duplicates :
10 12 11 11 12 11 10
Linked list after removing duplicates :
10 12 11 ```

Thanks to bearwang for suggesting this method.

Time Complexity: O(N), on average (assuming that hash table access time is O(1) on average).
Auxiliary Space : O(N), As extra space is used to store the elements in the stack.

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