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Remove all occurrences of duplicates from a sorted Linked List
• Difficulty Level : Medium
• Last Updated : 02 Sep, 2020

Given a sorted linked list, delete all nodes that have duplicate numbers (all occurrences), leaving only numbers that appear once in the original list.
Examples:

```Input : 23->28->28->35->49->49->53->53
Output : 23->35

Input : 11->11->11->11->75->75
Output : empty List
```

Note that this is different from Remove Duplicates From Linked List

The idea is to maintain a pointer (prev) to the node which just previous to the block of nodes we are checking for duplicates. In the first example, the pointer prev would point to 23 while we check for duplicates for the node 28. Once we reach the last duplicate node with value 28 (name it current pointer), we can make the next field of prev node to be the next of current and update current=current.next. This would delete the block of nodes with value 28 which has duplicates.

## C++

 `// C++ program to remove all ``// occurrences of duplicates ``// from a sorted 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 print nodes ``// in a given linked list.``void` `printList(``struct` `Node *node)``{``    ``while` `(node != NULL)``    ``{``        ``printf``(``"%d "``, node -> data);``        ``node = node -> next;``    ``}``}`` ` `// Function to remove all occurrences``// of duplicate elements``void` `removeAllDuplicates(``struct` `Node* &start)``{``    ``// create a dummy node ``    ``// that acts like a fake``    ``// head of list pointing``    ``// to the original head``    ``Node* dummy = ``new` `Node;`` ` `    ``// dummy node points``    ``// to the original head``    ``dummy -> next = start;``     ` `    ``// Node pointing to last ``    ``// node which has no duplicate.``    ``Node* prev = dummy;``     ` `    ``// Node used to traverse``    ``// the linked list.``    ``Node* current = start;`` ` `    ``while``(current != NULL)``    ``{``        ``// Until the current and ``        ``// previous values are ``        ``// same, keep updating current``        ``while``(current -> next != NULL &&``              ``prev -> next -> data == current -> next -> data)``            ``current = current -> next;`` ` `        ``// if current has unique value ``        ``// i.e current is not updated, ``        ``// Move the prev pointer to ``        ``// next node``        ``if` `(prev -> next == current)``            ``prev = prev -> next;`` ` `        ``// when current is updated ``        ``// to the last duplicate ``        ``// value of that segment, ``        ``// make prev the next of current``        ``else``            ``prev -> next = current -> next;`` ` `        ``current = current -> next;``    ``}`` ` `    ``// update original head to ``    ``// the next of dummy node ``    ``start = dummy -> next;``}`` ` `// Driver Code``int` `main() ``{``    ``// 23->28->28->35->49->49->53->53``    ``struct` `Node* start = newNode(23);``    ``start -> next = newNode(28);``    ``start -> next -> next = newNode(28);``    ``start -> next -> ``     ``next -> next = newNode(35);``    ``start -> next -> ``    ``next -> next -> next = newNode(49);``    ``start -> next -> ``    ``next -> next -> ``    ``next -> next = newNode(49);``    ``start -> next -> ``    ``next -> next -> ``    ``next -> next -> next = newNode(53);``    ``start -> next -> ``    ``next -> next -> ``    ``next -> next -> ``    ``next -> next = newNode(53);``    ``cout << ``"List before removal "` `<<``                  ``"of duplicates\n"``;``    ``printList(start);``     ` `    ``removeAllDuplicates(start);``     ` `    ``cout << ``"\nList after removal "` `<< ``                   ``"of duplicates\n"``;``    ``printList(start);``    ``return` `0;``}`` ` `// This code is contributed ``// by NIKHIL JINDAL`

## Java

 `// Java program to remove all occurrences of``// duplicates from a sorted linked list `` ` `// class to create Linked lIst ``class` `LinkedList{``     ` `// head of linked list ``Node head = ``null``; ``class` `Node``{``     ` `    ``// value in the node ``    ``int` `val; ``    ``Node next;``    ``Node(``int` `v)``    ``{``         ` `        ``// default value of the next``        ``// pointer field ``        ``val = v;``        ``next = ``null``;``    ``}``}`` ` `// Function to insert data nodes into``// the Linked List at the front``public` `void` `insert(``int` `data)``{``    ``Node new_node = ``new` `Node(data);``    ``new_node.next = head;``    ``head = new_node;``}`` ` `// Function to remove all occurrences``// of duplicate elements ``public` `void` `removeAllDuplicates()``{``     ` `    ``// Create a dummy node that acts like a fake``    ``// head of list pointing to the original head``    ``Node dummy = ``new` `Node(``0``);`` ` `    ``// Dummy node points to the original head``    ``dummy.next = head;``    ``Node prev = dummy;``    ``Node current = head;`` ` `    ``while` `(current != ``null``)``    ``{``        ``// Until the current and previous values``        ``// are same, keep updating current ``        ``while` `(current.next != ``null` `&&``               ``prev.next.val == current.next.val)``               ``current = current.next;`` ` `        ``// If current has unique value i.e current``        ``// is not updated, Move the prev pointer``        ``// to next node``        ``if` `(prev.next == current)``            ``prev = prev.next;`` ` `        ``// When current is updated to the last``        ``// duplicate value of that segment, make``        ``// prev the next of current*/``        ``else``            ``prev.next = current.next;`` ` `        ``current = current.next;``    ``}`` ` `    ``// Update original head to the next of dummy``    ``// node ``    ``head = dummy.next;``}`` ` `// Function to print the list elements``public` `void` `printList()``{``    ``Node trav = head;``    ``if` `(head == ``null``)``        ``System.out.print(``" List is empty"` `);``         ` `    ``while` `(trav != ``null``)``    ``{``        ``System.out.print(trav.val + ``" "``);``        ``trav = trav.next;``    ``}``}`` ` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``LinkedList ll = ``new` `LinkedList();``    ``ll.insert(``53``);``    ``ll.insert(``53``);``    ``ll.insert(``49``);``    ``ll.insert(``49``);``    ``ll.insert(``35``);``    ``ll.insert(``28``);``    ``ll.insert(``28``);``    ``ll.insert(``23``);``     ` `    ``System.out.println(``"Before removal of duplicates"``);``    ``ll.printList();`` ` `    ``ll.removeAllDuplicates();`` ` `    ``System.out.println(``"\nAfter removal of duplicates"``);``    ``ll.printList();``}``}`

## Python3

 `# Python3 implementation for the above approach`` ` `# Creating node``class` `Node:``    ``def` `__init__(``self``, val):``        ``self``.val ``=` `val``        ``self``.``next` `=` `None``class` `LinkedList:``    ``def` `__init__(``self``):``        ``self``.head ``=` `None``         ` `    ``# add node into beganing of linked list``    ``def` `push(``self``, new_data):``        ``new_node ``=` `Node(new_data)``        ``new_node.``next` `=` `self``.head``        ``self``.head ``=` `new_node``        ``return` `new_node``         ` `    ``# Function to remove all occurrences``    ``# of duplicate elements``    ``def` `removeAllDuplicates(``self``, temp):``         ` `        ``# temp is head node of linkedlist``        ``curr ``=` `temp``        ``# print(' print something')``        ``head ``=` `prev ``=` `Node(``None``)``        ``head.``next` `=` `curr`` ` `        ``# Here we use same as we do in removing ``        ``# duplicates and only extra thing is that``        ``# we need to remove all elements ``        ``# having duplicates that we did in 30-31``        ``while` `curr ``and` `curr.``next``:``             ` `            ``# until the current value and next ``            ``# value are same keep updating the current value``            ``if` `curr.val ``=``=` `curr.``next``.val:``                ``while``(curr ``and` `curr.``next` `and` `                      ``curr.val ``=``=` `curr.``next``.val):``                    ``curr ``=` `curr.``next``                     ` `                    ``# still one of duplicate values left``                    ``# so point prec to curr``                ``curr ``=` `curr.``next``                ``prev.``next` `=` `curr``            ``else``:``                ``prev ``=` `prev.``next``                ``curr ``=` `curr.``next``        ``return` `head.``next``         ` `    ``# for print the linkedlist``    ``def` `printList(``self``):``        ``temp1 ``=` `self``.head``        ``while` `temp1 ``is` `not` `None``:``            ``print``(temp1.val, end ``=` `" "``)``            ``temp1 ``=` `temp1.``next``             ` `    ``# For getting head of linkedlist``    ``def` `get_head(``self``):``        ``return` `self``.head`` ` `# Driver Code``if` `__name__``=``=``'__main__'``:``    ``llist ``=` `LinkedList()``    ``llist.push(``53``)``    ``llist.push(``53``)``    ``llist.push(``49``)``    ``llist.push(``49``)``    ``llist.push(``35``)``    ``llist.push(``28``)``    ``llist.push(``28``)``    ``llist.push(``23``)``     ` `    ``print``(``'Created linked list is:'``)``    ``llist.printList()``    ``print``(``'\nLinked list after deletion of duplicates:'``)``    ``head1 ``=` `llist.get_head()``    ``#print(head1)``    ``llist.removeAllDuplicates(head1)``    ``llist.printList()``         ` `# This code is contributed ``# by PRAVEEN KUMAR(IIIT KALYANI)`

## C#

 `/* C# program to remove all occurrences of``duplicates from a sorted linked list */`` ` `using` `System;`` ` `/* class to create Linked lIst */``public` `class` `LinkedList``{``    ``Node head = ``null``; ``/* head of linked list */``    ``class` `Node``    ``{``        ``public` `int` `val; ``/* value in the node */``        ``public` `Node next;``        ``public` `Node(``int` `v)``        ``{``            ``/* default value of the next``            ``pointer field */``            ``val = v;``            ``next = ``null``;``        ``}``    ``}`` ` `    ``/* Function to insert data nodes into``    ``the Linked List at the front */``    ``public` `void` `insert(``int` `data)``    ``{``        ``Node new_node = ``new` `Node(data);``        ``new_node.next = head;``        ``head = new_node;``    ``}`` ` `    ``/* Function to remove all occurrences``    ``of duplicate elements */``    ``public` `void` `removeAllDuplicates()``    ``{``    ``/* create a dummy node that acts like a fake``        ``head of list pointing to the original head*/``        ``Node dummy = ``new` `Node(0);`` ` `        ``/* dummy node points to the original head*/``        ``dummy.next = head;``        ``Node prev = dummy;``        ``Node current = head;`` ` `        ``while` `(current != ``null``)``        ``{``            ``/* Until the current and previous values``            ``are same, keep updating current */``            ``while` `(current.next != ``null` `&&``                ``prev.next.val == current.next.val)``                ``current = current.next;`` ` `            ``/* if current has unique value i.e current``                ``is not updated, Move the prev pointer``                ``to next node*/``            ``if` `(prev.next == current)``                ``prev = prev.next;`` ` `            ``/* when current is updated to the last``            ``duplicate value of that segment, make``            ``prev the next of current*/``            ``else``                ``prev.next = current.next;`` ` `            ``current = current.next;``        ``}`` ` `        ``/* update original head to the next of dummy``        ``node */``        ``head = dummy.next;``    ``}`` ` `    ``/* Function to print the list elements */``    ``public` `void` `printList()``    ``{``        ``Node trav = head;``        ``if` `(head == ``null``)``            ``Console.Write(``" List is empty"` `);``        ``while` `(trav != ``null``)``        ``{``            ``Console.Write(trav.val + ``" "``);``            ``trav = trav.next;``        ``}``    ``}`` ` `    ``/* Driver code */``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``LinkedList ll = ``new` `LinkedList();``        ``ll.insert(53);``        ``ll.insert(53);``        ``ll.insert(49);``        ``ll.insert(49);``        ``ll.insert(35);``        ``ll.insert(28);``        ``ll.insert(28);``        ``ll.insert(23);``        ``Console.WriteLine(``"Before removal of duplicates"``);``        ``ll.printList();`` ` `        ``ll.removeAllDuplicates();`` ` `        ``Console.WriteLine(``"\nAfter removal of duplicates"``);``        ``ll.printList();``    ``}``}`` ` `// This code is contributed by Rajput-Ji`

Output:

```List before removal of duplicates
23 28 28 35 49 49 53 53
List after removal of duplicates
23 35
```

Time Complexity : O(n)

This article is contributed by Saloni Baweja. 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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