Remove duplicates from a sorted linked list

01 Jul, 2022
Write a function that takes a list sorted in non-decreasing order and deletes any duplicate nodes from the list. The list should only be traversed once. For example if the linked list is 11->11->11->21->43->43->60 then removeDuplicates() should convert the list to 11->21->43->60. 
Algorithm: Traverse the list from the head (or start) node. While traversing, compare each node with its next node. If the data of the next node is the same as the current node then delete the next node. Before we delete a node, we need to store the next pointer of the node 
Implementation: Functions other than removeDuplicates() are just to create a linked list and test removeDuplicates(). 
Java
C
Java
Python3
C#
Javascript
/* C++ Program to remove duplicates from a sorted linked list */#include <bits/stdc++.h>using namespace std; /* Link list node */class Node{    public:    int data;    Node* next;}; /* The function removes duplicates from a sorted list */void removeDuplicates(Node* head){    /* Pointer to traverse the linked list */    Node* current = head;     /* Pointer to store the next pointer of a node to be deleted*/    Node* next_next;         /* do nothing if the list is empty */    if (current == NULL)    return;     /* Traverse the list till last node */    while (current->next != NULL)    {    /* Compare current node with next node */    if (current->data == current->next->data)    {        /* The sequence of steps is important*/               next_next = current->next->next;        free(current->next);        current->next = next_next;    }    else /* This is tricky: only advance if no deletion */    {        current = current->next;    }    }} /* UTILITY FUNCTIONS *//* Function to insert a node at the beginning of the linked list */void push(Node** head_ref, int new_data){    /* allocate node */    Node* new_node = new Node();                 /* put in the data */    new_node->data = new_data;                     /* link the old list off the new node */    new_node->next = (*head_ref);                 /* move the head to point to the new node */    (*head_ref) = new_node;} /* Function to print nodes in a given linked list */void printList(Node *node){    while (node!=NULL)    {        cout<<" "<<node->data;        node = node->next;    }} /* Driver program to test above functions*/int main(){    /* Start with the empty list */    Node* head = NULL;         /* Let us create a sorted linked list to test the functions    Created linked list will be 11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);    push(&head, 11);    push(&head, 11);    push(&head, 11);                                         cout<<"Linked list before duplicate removal ";    printList(head);     /* Remove duplicates from linked list */    removeDuplicates(head);     cout<<"\nLinked list after duplicate removal ";        printList(head);                     return 0;} // This code is contributed by rathbhupendra
/* C Program to remove duplicates from a sorted linked list */#include<stdio.h>#include<stdlib.h> /* Link list node */struct Node{    int data;    struct Node* next;}; /* The function removes duplicates from a sorted list */void removeDuplicates(struct Node* head){    /* Pointer to traverse the linked list */    struct Node* current = head;     /* Pointer to store the next pointer of a node to be deleted*/    struct Node* next_next;       /* do nothing if the list is empty */    if (current == NULL)       return;     /* Traverse the list till last node */    while (current->next != NULL)    {       /* Compare current node with next node */       if (current->data == current->next->data)       {           /* The sequence of steps is important*/                         next_next = current->next->next;           free(current->next);           current->next = next_next;        }       else /* This is tricky: only advance if no deletion */       {          current = current->next;       }    }} /* UTILITY FUNCTIONS *//* Function to insert a node at the beginning of the 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;                     /* link the old list off the new node */    new_node->next = (*head_ref);                 /* move the head to point to the new node */    (*head_ref)    = new_node;} /* 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 functions*/int main(){    /* Start with the empty list */    struct Node* head = NULL;       /* Let us create a sorted linked list to test the functions     Created linked list will be 11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);     push(&head, 11);    push(&head, 11);    push(&head, 11);                                        printf("\n Linked list before duplicate removal  ");    printList(head);     /* Remove duplicates from linked list */    removeDuplicates(head);     printf("\n Linked list after duplicate removal ");            printList(head);                  return 0;}
// Java program to remove duplicates from a sorted linked listclass LinkedList{    Node head;  // head of list      /* Linked list Node*/    class Node    {        int data;        Node next;        Node(int d) {data = d; next = null; }    }     void removeDuplicates()    {        /*Another reference to head*/        Node curr = head;         /* Traverse list till the last node */        while (curr != null) {             Node temp = curr;            /*Compare current node with the next node and            keep on deleting them until it matches the current            node data */            while(temp!=null && temp.data.equals(curr.data)) {                temp = temp.next;            }            /*Set current node next to the next different            element denoted by temp*/            curr.next = temp;            curr = curr.next;        }    }                         /* Utility functions */     /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }     /* Function to print linked list */     void printList()     {         Node temp = head;         while (temp != null)         {            System.out.print(temp.data+" ");            temp = temp.next;         }          System.out.println();     }      /* Driver program to test above functions */    public static void main(String args[])    {        LinkedList llist = new LinkedList();        llist.push(20);        llist.push(13);        llist.push(13);        llist.push(11);        llist.push(11);        llist.push(11);                 System.out.println("List before removal of duplicates");        llist.printList();                 llist.removeDuplicates();                 System.out.println("List after removal of elements");        llist.printList();    }}/* This code is contributed by Rajat Mishra */   
# Python3 program to remove duplicate# nodes from a sorted linked list # Node classclass Node:     # Constructor to initialize    # the node object    def __init__(self, data):        self.data = data        self.next = None class LinkedList:     # Function to initialize head    def __init__(self):        self.head = None     # Function to insert a new node    # at the beginning    def push(self, new_data):        new_node = Node(new_data)        new_node.next = self.head        self.head = new_node     # Given a reference to the head of a    # list and a key, delete the first    # occurrence of key in linked list    def deleteNode(self, key):                 # Store head node        temp = self.head         # If head node itself holds the        # key to be deleted        if (temp is not None):            if (temp.data == key):                self.head = temp.next                temp = None                return         # Search for the key to be deleted,        # keep track of the previous node as        # we need to change 'prev.next'        while(temp is not None):            if temp.data == key:                break            prev = temp            temp = temp.next         # if key was not present in        # linked list        if(temp == None):            return         # Unlink the node from linked list        prev.next = temp.next         temp = None     # Utility function to print the    # linked LinkedList    def printList(self):        temp = self.head        while(temp):            print(temp.data , end = ' ')            temp = temp.next         # This function removes duplicates    # from a sorted list            def removeDuplicates(self):        temp = self.head        if temp is None:            return        while temp.next is not None:            if temp.data == temp.next.data:                new = temp.next.next                temp.next = None                temp.next = new            else:                temp = temp.next        return self.head # Driver Codellist = LinkedList() llist.push(20)llist.push(13)llist.push(13)llist.push(11)llist.push(11)llist.push(11)print ("Created Linked List: ")llist.printList()print()print("Linked List after removing",             "duplicate elements:")llist.removeDuplicates()llist.printList() # This code is contributed by# Dushyant Pathak.
// C# program to remove duplicates// from a sorted linked listusing System;     public class LinkedList{    Node head; // head of list     /* Linked list Node*/    class Node    {        public int data;        public Node next;        public Node(int d)        {            data = d; next = null;        }    }     void removeDuplicates()    {        /*Another reference to head*/        Node current = head;         /* Pointer to store the next        pointer of a node to be deleted*/        Node next_next;         /* do nothing if the list is empty */        if (head == null)            return;         /* Traverse list till the last node */        while (current.next != null)        {             /*Compare current node with the next node */            if (current.data == current.next.data)            {                next_next = current.next.next;                current.next = null;                current.next = next_next;            }            else // advance if no deletion                current = current.next;        }    }                         /* Utility functions */     /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                Put in the data*/        Node new_node = new Node(new_data);         /* 3. Make next of new Node as head */        new_node.next = head;         /* 4. Move the head to point to new Node */        head = new_node;    }     /* Function to print linked list */    void printList()    {        Node temp = head;        while (temp != null)        {            Console.Write(temp.data+" ");            temp = temp.next;        }        Console.WriteLine();    }     /* Driver code */    public static void Main(String []args)    {        LinkedList llist = new LinkedList();        llist.push(20);        llist.push(13);        llist.push(13);        llist.push(11);        llist.push(11);        llist.push(11);                 Console.WriteLine("List before removal of duplicates");        llist.printList();                 llist.removeDuplicates();                 Console.WriteLine("List after removal of elements");        llist.printList();    }} /* This code is contributed by 29AjayKumar */
<script>// Javascript program to remove duplicates from a sorted linked list          /* Linked list Node*/    class Node    {        constructor(d) {            this.data = d;            this.next = null;          }    }         let head=new Node();    // head of list    function removeDuplicates()    {        /*Another reference to head*/        let curr = head;          /* Traverse list till the last node */        while (curr != null) {             let temp = curr;            /*Compare current node with the next node and            keep on deleting them until it matches the current            node data */            while(temp!=null && temp.data==curr.data) {                temp = temp.next;            }            /*Set current node next to the next different            element denoted by temp*/            curr.next = temp;            curr = curr.next;        }    }         /* Utility functions */      /* Inserts a new Node at front of the list. */    function push(new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        let new_node = new Node(new_data);           /* 3. Make next of new Node as head */        new_node.next = head;           /* 4. Move the head to point to new Node */        head = new_node;    }         /* Function to print linked list */    function printList()    {        let temp = head;         while (temp != null && temp.data)         {                          document.write(temp.data+" ");            temp = temp.next;         }         document.write("<br>");    }         /* Driver program to test above functions */    push(20)    push(13)    push(13)    push(11)    push(11)    push(11)         document.write("List before removal of duplicates ");    printList();              removeDuplicates();              document.write("List after removal of elements ");    printList();              // This code is contributed by unknown2108</script>
Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20
Time Complexity: O(n) where n is the number of nodes in the given linked list.
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English
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Space Complexity: O(1) , as there is no extra space used.
Recursive Approach :  
C++
C
Java
Python3
C#
Javascript
/* C++ Program to remove duplicatesfrom a sorted linked list */#include <bits/stdc++.h>using namespace std; /* Link list node */class Node{    public:    int data;    Node* next;}; /* The function removes duplicatesfrom a sorted list */void removeDuplicates(Node* head){    /* Pointer to store the pointer of a node to be deleted*/    Node* to_free;         /* do nothing if the list is empty */    if (head == NULL)        return;     /* Traverse the list till last node */    if (head->next != NULL)    {                 /* Compare head node with next node */        if (head->data == head->next->data)        {            /* The sequence of steps is important.              to_free pointer stores the next of head             pointer which is to be deleted.*/               to_free = head->next;        head->next = head->next->next;        free(to_free);        removeDuplicates(head);        }        else /* This is tricky: only        advance if no deletion */        {            removeDuplicates(head->next);        }    }} /* UTILITY FUNCTIONS *//* Function to insert a node at thebeginning of the linked list */void push(Node** head_ref, int new_data){    /* allocate node */    Node* new_node = new Node();                 /* put in the data */    new_node->data = new_data;                     /* link the old list off the new node */    new_node->next = (*head_ref);                 /* move the head to point to the new node */    (*head_ref) = new_node;} /* Function to print nodesin a given linked list */void printList(Node *node){    while (node!=NULL)    {        cout<<" "<<node->data;        node = node->next;    }} /* Driver code*/int main(){    /* Start with the empty list */    Node* head = NULL;         /* Let us create a sorted linked    list to test the functions    Created linked list will be    11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);    push(&head, 11);    push(&head, 11);    push(&head, 11);                                         cout<<"Linked list before duplicate removal ";    printList(head);     /* Remove duplicates from linked list */    removeDuplicates(head);     cout<<"\nLinked list after duplicate removal ";        printList(head);                     return 0;}// This code is contributed by Ashita Gupta
/* C recursive Program to remove duplicates from a sorted linked list */#include<stdio.h>#include<stdlib.h> /* Link list node */struct Node{    int data;    struct Node* next;};/* UTILITY FUNCTIONS *//* Function to insert a node at the beginning of the 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;                     /* link the old list off the new node */    new_node->next = (*head_ref);                 /* move the head to point to the new node */    (*head_ref)    = new_node;} /* Function to print nodes in a given linked list */void printList(struct Node *node){    while (node!=NULL)    {       printf("%d ", node->data);       node = node->next;    }}  Node* deleteDuplicates(Node* head) {        if (head == nullptr) return nullptr;        if (head->next == nullptr) return head;         if (head->data == head->next->data) {            Node *tmp;            // If find next element duplicate, preserve the next pointer to be deleted,            // skip it, and then delete the stored one. Return head            tmp = head->next;            head->next = head->next->next;            free(tmp);            return deleteDuplicates(head);        }         else {             // if doesn't find next element duplicate, leave head            // and check from next element            head->next = deleteDuplicates(head->next);            return head;        }    } int main(){    /* Start with the empty list */    struct Node* head = NULL;       /* Let us create a sorted linked list to test the functions     Created linked list will be 11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);     push(&head, 11);    push(&head, 11);    push(&head, 11);                                        printf("\n Linked list before duplicate removal  ");    printList(head);     /* Remove duplicates from linked list */    head = deleteDuplicates(head);     printf("\n Linked list after duplicate removal ");            printList(head);                  return 0;}/* This code is contributed by Yogesh shukla */  
// Java Program to remove duplicates// from a sorted linked listclass GFG{/* Link list node */static class Node{    int data;    Node next;}; // The function removes duplicates// from a sorted liststatic Node removeDuplicates(Node head){    /* Pointer to store the pointer    of a node to be deleted*/    Node to_free;         /* do nothing if the list is empty */    if (head == null)        return null;     /* Traverse the list till last node */    if (head.next != null)    {                 /* Compare head node with next node */        if (head.data == head.next.data)        {            /* The sequence of steps is important.            to_free pointer stores the next of head            pointer which is to be deleted.*/            to_free = head.next;            head.next = head.next.next;            removeDuplicates(head);        }                 /* This is tricky: only advance if no deletion */        else        {            removeDuplicates(head.next);        }    }    return head;} /* UTILITY FUNCTIONS *//* Function to insert a node at the beginningof the 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;                     /* link the old list off the new node */    new_node.next = (head_ref);                 /* move the head to point to the new node */    (head_ref) = new_node;    return head_ref;} /* Function to print nodes in a given linked list */static void printList(Node node){    while (node != null)    {        System.out.print(" " + node.data);        node = node.next;    }} /* Driver code*/public static void main(String args[]){    /* Start with the empty list */    Node head = null;         /* Let us create a sorted linked list    to test the functions    Created linked list will be 11.11.11.13.13.20 */    head = push(head, 20);    head = push(head, 13);    head = push(head, 13);    head = push(head, 11);    head = push(head, 11);    head = push(head, 11);                                         System.out.println("Linked list before" +                      " duplicate removal ");    printList(head);     /* Remove duplicates from linked list */    head = removeDuplicates(head);     System.out.println("\nLinked list after" +                       " duplicate removal ");        printList(head);            }}  // This code is contributed by Arnab Kundu
# Python3 Program to remove duplicates# from a sorted linked listimport math # Link list nodeclass Node:    def __init__(self,data):        self.data = data        self.next = None # The function removes duplicates# from a sorted listdef removeDuplicates(head):         # Pointer to store the pointer of a node    # to be deleted to_free         # do nothing if the list is empty    if (head == None):        return     # Traverse the list till last node    if (head.next != None):                 # Compare head node with next node        if (head.data == head.next.data):                         # The sequence of steps is important.            # to_free pointer stores the next of head            # pointer which is to be deleted.            to_free = head.next            head.next = head.next.next                         # free(to_free)            removeDuplicates(head)                 # This is tricky: only advance if no deletion        else:            removeDuplicates(head.next)             return head # UTILITY FUNCTIONS# Function to insert a node at the# beginning of the linked listdef push(head_ref, new_data):         # allocate node    new_node = Node(new_data)                 # put in the data    new_node.data = new_data                     # link the old list off the new node    new_node.next = head_ref                 # move the head to point to the new node    head_ref = new_node    return head_ref # Function to print nodes in a given linked listdef printList(node):    while (node != None):        print(node.data, end = " ")        node = node.next     # Driver codeif __name__=='__main__':     # Start with the empty list    head = None         # Let us create a sorted linked list    # to test the functions    # Created linked list will be 11.11.11.13.13.20    head = push(head, 20)    head = push(head, 13)    head = push(head, 13)    head = push(head, 11)    head = push(head, 11)    head = push(head, 11)                                     print("Linked list before duplicate removal ",                                         end = "")    printList(head)     # Remove duplicates from linked list    removeDuplicates(head)     print("\nLinked list after duplicate removal ",                                          end = "")    printList(head)             # This code is contributed by Srathore
// C# Program to remove duplicates// from a sorted linked listusing System;     class GFG{/* Link list node */public class Node{    public int data;    public Node next;}; // The function removes duplicates// from a sorted liststatic Node removeDuplicates(Node head){    /* Pointer to store the pointer    of a node to be deleted*/    Node to_free;         /* do nothing if the list is empty */    if (head == null)        return null;     /* Traverse the list till last node */    if (head.next != null)    {                 /* Compare head node with next node */        if (head.data == head.next.data)        {            /* The sequence of steps is important.            to_free pointer stores the next of head            pointer which is to be deleted.*/            to_free = head.next;            head.next = head.next.next;            removeDuplicates(head);        }                 /* This is tricky: only advance if no deletion */        else        {            removeDuplicates(head.next);        }    }    return head;} /* UTILITY FUNCTIONS *//* Function to insert a node at the beginningof the 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;                     /* link the old list off the new node */    new_node.next = (head_ref);                 /* move the head to point to the new node */    (head_ref) = new_node;    return head_ref;} /* Function to print nodes ina given linked list */static void printList(Node node){    while (node != null)    {        Console.Write(" " + node.data);        node = node.next;    }} // Driver codepublic static void Main(String []args){    /* Start with the empty list */    Node head = null;         /* Let us create a sorted linked list    to test the functions    Created linked list will be 11.11.11.13.13.20 */    head = push(head, 20);    head = push(head, 13);    head = push(head, 13);    head = push(head, 11);    head = push(head, 11);    head = push(head, 11);                                         Console.Write("Linked list before" +                  " duplicate removal ");    printList(head);     /* Remove duplicates from linked list */    head = removeDuplicates(head);     Console.Write("\nLinked list after" +                  " duplicate removal ");        printList(head);            }} // This code is contributed by PrinciRaj1992
<script> // JavaScript Program to remove duplicates// from a sorted linked list     /* Link list node */class Node {    constructor(val) {        this.data = val;        this.next = null;    }}    // The function removes duplicates    // from a sorted list    function removeDuplicates(head) {        /*         Pointer to store the pointer         of a node to be deleted         */       var to_free;         /* do nothing if the list is empty */        if (head == null)            return null;         /* Traverse the list till last node */        if (head.next != null) {             /* Compare head node with next node */            if (head.data == head.next.data) {                /*                 The sequence of steps is important.                 to_free pointer stores the next of head                 pointer which is to be deleted.                 */                to_free = head.next;                head.next = head.next.next;                removeDuplicates(head);            }             /* This is tricky: only advance            if no deletion */            else {                removeDuplicates(head.next);            }        }        return head;    }     /* UTILITY FUNCTIONS */    /*     Function to insert a node at     the beginning of the linked list     */    function push(head_ref , new_data) {        /* allocate node */var new_node = new Node();         /* put in the data */        new_node.data = new_data;         /* link the old list off the new node */        new_node.next = (head_ref);         /* move the head to point to the new node */        (head_ref) = new_node;        return head_ref;    }     /* Function to print nodes in a given linked list */    function printList(node) {        while (node != null) {            document.write(" " + node.data);            node = node.next;        }    }     /* Driver code */             /* Start with the empty list */var head = null;         /*          Let us create a sorted linked list to         test the functions Created linked list          will be 11.11.11.13.13.20         */        head = push(head, 20);        head = push(head, 13);        head = push(head, 13);        head = push(head, 11);        head = push(head, 11);        head = push(head, 11);         document.write("Linked list before" +        " duplicate removal <br/>");        printList(head);         /* Remove duplicates from linked list */        head = removeDuplicates(head);         document.write("<br/>Linked list after" +        " duplicate removal <br/>");        printList(head); // This code is contributed by todaysgaurav </script>
Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20
Time Complexity: O(n) where n is the number of nodes in the given linked list.
Auxiliary Space: O(n)
Another Approach: Create a pointer that will point towards the first occurrence of every element and another pointer temp which will iterate to every element and when the value of the previous pointer is not equal to the temp pointer, we will set the pointer of the previous pointer to the first occurrence of another node.
Below is the implementation of the above approach:
C++14
C
Java
Python3
C#
Javascript
// C++ program to remove duplicates// from a sorted linked list#include <bits/stdc++.h>using namespace std; // Linked list Nodestruct Node {    int data;    Node* next;    Node(int d)    {        data = d;        next = NULL;    }}; // Function to remove duplicates// from the given linked listNode* removeDuplicates(Node* head){    // Two references to head temp will iterate to the whole    // Linked List prev will point towards the first    // occurrence of every element    Node *temp = head, *prev = head;    // Traverse list till the last node    while (temp != NULL) {        // Compare values of both pointers        if (temp->data != prev->data) {            // if the value of prev is not equal to the            // value of temp that means there are no more            // occurrences of the prev data-> So we can set            // the next of prev to the temp node->*/            prev->next = temp;            prev = temp;        }        //Set the temp to the next node        temp = temp->next;    }    // This is the edge case if there are more than one    // occurrences of the last element    if (prev != temp)        prev->next = NULL;    return head;} Node* push(Node* head, int new_data){    /* 1 & 2: Allocate the Node & Put in the data*/    Node* new_node = new Node(new_data);    /* 3. Make next of new Node as head */    new_node->next = head;    /* 4. Move the head to point to new Node */    head = new_node;    return head;} /* Function to print linked list */void printList(Node* head){    Node* temp = head;    while (temp != NULL) {        cout << temp->data << " ";        temp = temp->next;    }    cout << endl;} /* Driver code */int main(){    Node* llist = NULL;    llist = push(llist, 20);    llist = push(llist, 13);    llist = push(llist, 13);    llist = push(llist, 11);    llist = push(llist, 11);    llist = push(llist, 11);    cout << ("List before removal of duplicates\n");    printList(llist);    cout << ("List after removal of elements\n");    llist = removeDuplicates(llist);    printList(llist);} // This code is contributed by Sania Kumari Gupta
// C program to remove duplicates from a sorted linked list#include <stdio.h>#include <stdlib.h> /* Link list node */typedef struct Node {    int data;    struct Node* next;} Node; // Function to remove duplicates// from the given linked listNode* removeDuplicates(Node* head){    // Two references to head temp will iterate to the whole    // Linked List prev will point towards the first    // occurrence of every element    Node *temp = head, *prev = head;    // Traverse list till the last node    while (temp != NULL) {        // Compare values of both pointers        if (temp->data != prev->data) {            // if the value of prev is not equal to the            // value of temp that means there are no more            // occurrences of the prev data-> So we can set            // the next of prev to the temp node->*/            prev->next = temp;            prev = temp;        }        // Set the temp to the next node        temp = temp->next;    }    // This is the edge case if there are more than one    // occurrences of the last element    if (prev != temp)        prev->next = NULL;    return head;} void push(Node** head_ref, int new_data){    /* allocate node */    Node* new_node = (Node*)malloc(sizeof(Node));    /* put in the data  */    new_node->data = new_data;    /* link the old list off the new node */    new_node->next = (*head_ref);    /* move the head to point to the new node */    (*head_ref) = new_node;} /* Function to print linked list */void printList(Node* head){    Node* temp = head;    while (temp != NULL) {        printf("%d ", temp->data);        temp = temp->next;    }    printf("\n");} /* Driver code */int main(){    Node* llist = NULL;    push(&llist, 20);    push(&llist, 13);    push(&llist, 13);    push(&llist, 11);    push(&llist, 11);    push(&llist, 11);    printf("List before removal of duplicates\n");    printList(llist);    printf("List after removal of elements\n");    llist = removeDuplicates(llist);    printList(llist);} // This code is contributed by Sania Kumari Gupta
// Java program to remove duplicates// from a sorted linked listclass LinkedList {    // head of list    Node head;    // Linked list Node    class Node {        int data;        Node next;        Node(int d)        {            data = d;            next = null;        }    }     // Function to remove duplicates from the given linked    // list    void removeDuplicates()    {        // Two references to head temp will iterate to the        // whole Linked List prev will point towards the        // first occurrence of every element        Node temp = head, prev = head;         // Traverse list till the last node        while (temp != null) {             // Compare values of both pointers            if (temp.data != prev.data) {                // if the value of prev is not equal to the                // value of temp that means there are no                // more occurrences of the prev data. So we                // can set the next of prev to the temp                // node.                prev.next = temp;                prev = temp;            }            //Set the temp to the next node            temp = temp.next;        }        // This is the edge case if there are more than one        // occurrences of the last element        if (prev != temp)            prev.next = null;    }     /* Utility functions */     /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        Node new_node = new Node(new_data);        /* 3. Make next of new Node as head */        new_node.next = head;        /* 4. Move the head to point to new Node */        head = new_node;    }     /* Function to print linked list */    void printList()    {        Node temp = head;        while (temp != null) {            System.out.print(temp.data + " ");            temp = temp.next;        }        System.out.println();    }     /* Driver program to test above functions */    public static void main(String args[])    {        LinkedList llist = new LinkedList();        llist.push(20);        llist.push(13);        llist.push(13);        llist.push(11);        llist.push(11);        llist.push(11);         System.out.print("List before ");        System.out.println("removal of duplicates");        llist.printList();         llist.removeDuplicates();         System.out.println(            "List after removal of elements");        llist.printList();    }} // This code is contributed by Sania Kumari Gupta
# Python3 program to remove duplicates# from a sorted linked list import math   # Link list node class Node:          def __init__(self, data):                 self.data = data         self.next = None   # The function removes duplicates # from the given linked listdef removeDuplicates(head):          # Do nothing if the list consist of    # only one element or empty     if (head == None and        head.next == None):        return         # Construct a pointer    # pointing towards head    current = head         # Initialise a while loop till the    # second last node of the linkedlist    while (current.next):                 # If the data of current and next        # node is equal we will skip the        # node between them        if current.data == current.next.data:            current.next = current.next.next                     # If the data of current and        # next node is different move        # the pointer to the next node        else:            current = current.next         return   # UTILITY FUNCTIONS # Function to insert a node at the # beginning of the linked list def push(head_ref, new_data):          # Allocate node     new_node = Node(new_data)                    # Put in the data     new_node.data = new_data                        # Link the old list off    # the new node     new_node.next = head_ref                    # Move the head to point    # to the new node     head_ref = new_node         return head_ref   # Function to print nodes# in a given linked list def printList(node):          while (node != None):         print(node.data, end = " ")         node = node.next       # Driver codeif __name__=='__main__':        head = None         head = push(head, 20)     head = push(head, 13)     head = push(head, 13)     head = push(head, 11)     head = push(head, 11)     head = push(head, 11)                                        print("List before removal of "           "duplicates ", end = "")     printList(head)        removeDuplicates(head)        print("\nList after removal of "           "elements ", end = "")                printList(head)                # This code is contributed by MukulTomar
// C# program to remove duplicates// from a sorted linked listusing System; class LinkedList{       // head of list    public Node head;      // Linked list Node    public class Node    {        public int data;        public Node next;        public Node(int d) {          data = d;          next = null;        }    }     // Function to remove duplicates    // from the given linked list    void removeDuplicates()    {               // Two references to head        // temp will iterate to the        // whole Linked List        // prev will point towards        // the first occurrence of every element        Node temp = head, prev = head;         // Traverse list till the last node        while (temp != null) {                       // Compare values of both pointers           if(temp.data != prev.data)           {                           /* if the value of prev is             not equal to the value of             temp that means there are no             more occurrences of the prev data.             So we can set the next of             prev to the temp node.*/             prev.next = temp;             prev = temp;           }                       /*Set the temp to the next node*/            temp = temp.next;        }             /*This is the edge case if there      are more than one occurrences      of the last element*/      if(prev != temp){            prev.next = null;        }    }                         /* Utility functions */     /* Inserts a new Node at front of the list. */    public void push(int new_data)    {        /* 1 & 2: Allocate the Node &                  Put in the data*/        Node new_node = new Node(new_data);          /* 3. Make next of new Node as head */        new_node.next = head;          /* 4. Move the head to point to new Node */        head = new_node;    }     /* Function to print linked list */     void printList()     {         Node temp = head;         while (temp != null)         {            Console.Write(temp.data + " ");            temp = temp.next;         }          Console.WriteLine();     }      /* Driver code */    public static void Main(string []args)    {        LinkedList llist = new LinkedList();        llist.push(20);        llist.push(13);        llist.push(13);        llist.push(11);        llist.push(11);        llist.push(11);              Console.Write("List before ");        Console.WriteLine("removal of duplicates");        llist.printList();              llist.removeDuplicates();              Console.WriteLine("List after removal of elements");        llist.printList();    }} // This code is contributed by rutvik_56
<script>// javascript program to remove duplicates// from a sorted linked list     // head of list    var head;     // Linked list Node     class Node {            constructor(val) {                this.data = val;                this.next = null;            }        }           // Function to remove duplicates    // from the given linked list    function removeDuplicates() {        // Two references to head        // temp will iterate to the        // whole Linked List        // prev will point towards        // the first occurrence of every element        var temp = head, prev = head;         // Traverse list till the last node        while (temp != null) {             // Compare values of both pointers            if (temp.data != prev.data) {                /*                 * if the value of prev is not equal to the value of temp that means there are                 * no more occurrences of the prev data. So we can set the next of prev to the                 * temp node.                 */                prev.next = temp;                prev = temp;            }            /* Set the temp to the next node */            temp = temp.next;        }        /*         * This is the edge case if there are more than one occurrences of the last         * element         */        if (prev != temp) {            prev.next = null;        }    }     /* Utility functions */     /* Inserts a new Node at front of the list. */    function push(new_data) {        /*         * 1 & 2: Allocate the Node & Put in the data         */        var new_node = new Node(new_data);         /* 3. Make next of new Node as head */        new_node.next = head;         /* 4. Move the head to point to new Node */        head = new_node;    }     /* Function to print linked list */    function printList() {        var temp = head;        while (temp != null) {            document.write(temp.data + " ");            temp = temp.next;        }        document.write("<br/>");    }     /* Driver program to test above functions */             push(20);        push(13);        push(13);        push(11);        push(11);        push(11);         document.write("List before ");        document.write("removal of duplicates<br/>");        printList();         removeDuplicates();         document.write("List after removal of elements<br/>");        printList(); // This code contributed by aashish1995</script>
List before removal of duplicates
11 11 11 13 13 20 
List after removal of elements
11 13 20 
Time Complexity: O(n) where n is the number of nodes in the given linked list.
Auxiliary Space: O(1) 
Another Approach: Using Maps
The idea is to push all the values in a map and printing its keys.
Below is the implementation of the above approach:
C++
Java
Python3
C#
Javascript
// CPP program for the above approach#include <bits/stdc++.h>using namespace std; /* Link list node */struct Node {    int data;    Node* next;    Node()    {        data = 0;        next = NULL;    }}; /* Function to insert a node at   the beginning of the linked * list */void push(Node** head_ref, int new_data){         /* allocate node */    Node* new_node = new Node();     /* put in the data */    new_node->data = new_data;     /* link the old list off    the new node */    new_node->next = (*head_ref);     /* move the head to point    to the new node */    (*head_ref) = new_node;} /* Function to print nodesin a given linked list */void printList(Node* node){    while (node != NULL) {        cout << node->data << " ";        node = node->next;    }} // Function to remove duplicatesvoid removeDuplicates(Node* head){    unordered_map<int, bool> track;    Node* temp = head;    while (temp) {        if (track.find(temp->data) == track.end()) {            cout << temp->data << " ";        }        track[temp->data] = true;        temp = temp->next;    }} // Driver Codeint main(){    Node* head = NULL;     /* Created linked list will be    11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);    push(&head, 11);    push(&head, 11);    push(&head, 11);     cout << "Linked list before duplicate removal ";    printList(head);     cout << "\nLinked list after duplicate removal ";    removeDuplicates(head);     return 0;} // This code is contributed by yashbeersingh42
// Java program for the above approachimport java.io.*;import java.util.*; class Node{    int data;    Node next;    Node()    {        data = 0;        next = null;    }}class GFG{         /* Function to insert a node at   the beginning of the 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;                 /* link the old list off        the new node */        new_node.next = (head_ref);                 /* move the head to point        to the new node */        head_ref = new_node;        return head_ref;    }         /* Function to print nodes    in a given linked list */    static void printList(Node node)    {        while (node != null)        {            System.out.print(node.data + " ");            node = node.next;        }    }         // Function to remove duplicates    static void removeDuplicates(Node head)    {        HashMap<Integer, Boolean> track = new HashMap<>();        Node temp = head;                 while(temp != null)        {            if(!track.containsKey(temp.data))            {                System.out.print(temp.data + " ");            }            track.put(temp.data , true);            temp = temp.next;        }    }         // Driver Code    public static void main (String[] args)    {        Node head = null;               /* Created linked list will be        11->11->11->13->13->20 */        head = push(head, 20);        head = push(head, 13);        head = push(head, 13);        head = push(head, 11);        head = push(head, 11);        head = push(head, 11);        System.out.print("Linked list before duplicate removal ");        printList(head);        System.out.print("\nLinked list after duplicate removal  ");        removeDuplicates(head);    }} // This code is contributed by avanitrachhadiya2155
# Python program for the above approachclass Node:    def __init__(self):        self.data = 0;        self.next = None;     # Function to insert a node at# the beginning of the linked listdef push(head_ref, new_data):         # allocate node    new_node = Node();        # put in the data    new_node.data = new_data;          # link the old list off the new node    new_node.next = (head_ref);          # move the head to point to the new node    head_ref = new_node;    return head_ref; # Function to print nodes in a given linked listdef printList(node):    while (node != None):        print(node.data, end=" ");        node = node.next;      # Function to remove duplicatesdef removeDuplicates(head):    track = {};    temp = head;          while(temp != None):        if (not temp.data in track):            print(temp.data, end=" ");                 track[temp.data] = True;        temp = temp.next;  # Driver Codehead = None;        # Created linked list will be 11->11->11->13->13->20head = push(head, 20);head = push(head, 13);head = push(head, 13);head = push(head, 11);head = push(head, 11);head = push(head, 11);print("Linked list before duplicate removal ", end=" ");printList(head);print("\nLinked list after duplicate removal  ", end=" ");removeDuplicates(head); # This code is contributed by _Saurabh_jaiswal
// C# program for the above approachusing System;using System.Collections.Generic; public class Node{    public int data;    public Node next;    public Node()    {        data = 0;        next = null;    }}public class GFG{         /* Function to insert a node at   the beginning of the 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;                  /* link the old list off        the new node */        new_node.next = (head_ref);                  /* move the head to point        to the new node */        head_ref = new_node;        return head_ref;    }          /* Function to print nodes    in a given linked list */    static void printList(Node node)    {        while (node != null)        {            Console.Write(node.data + " ");            node = node.next;        }    }          // Function to remove duplicates    static void removeDuplicates(Node head)    {        Dictionary<int, bool> track = new Dictionary<int, bool>();        Node temp = head;        while(temp != null)        {            if(!track.ContainsKey(temp.data))            {                Console.Write(temp.data + " ");                track.Add(temp.data , true);            }                         temp = temp.next;        }    }       // Driver Code    static public void Main ()    {        Node head = null;                /* Created linked list will be        11->11->11->13->13->20 */        head = push(head, 20);        head = push(head, 13);        head = push(head, 13);        head = push(head, 11);        head = push(head, 11);        head = push(head, 11);                 Console.Write("Linked list before duplicate removal ");        printList(head);        Console.Write("\nLinked list after duplicate removal  ");        removeDuplicates(head);    }} // This code is contributed by rag2127
<script> // Javascript program for the above approachclass Node{    constructor()    {        this.data = 0;        this.next = null;    }} /* Function to insert a node at   the beginning of the linked * list */function push(head_ref, new_data){         /* allocate node */    let new_node = new Node();        /* put in the data */    new_node.data = new_data;          /* link the old list off    the new node */    new_node.next = (head_ref);          /* move the head to point    to the new node */    head_ref = new_node;    return head_ref;} /* Function to print nodesin a given linked list */function printList(node){    while (node != null)    {        document.write(node.data + " ");        node = node.next;    }} // Function to remove duplicatesfunction removeDuplicates(head){    let track = new Map();    let temp = head;          while(temp != null)    {        if (!track.has(temp.data))        {            document.write(temp.data + " ");        }        track.set(temp.data, true);        temp = temp.next;    }} // Driver Codelet head = null;        /* Created linked list will be11->11->11->13->13->20 */head = push(head, 20);head = push(head, 13);head = push(head, 13);head = push(head, 11);head = push(head, 11);head = push(head, 11);document.write("Linked list before duplicate removal ");printList(head);document.write("<br>Linked list after duplicate removal  ");removeDuplicates(head); // This code is contributed by patel2127 </script>
Linked list before duplicate removal 11 11 11 13 13 20 
Linked list after duplicate removal 11 13 20 
Time Complexity: O(n)  where n is the number of nodes in the given linked list.
Space Complexity: O(n) 
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