Remove duplicates from a sorted linked list

Write a function which 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 data of next node is same as current node then delete the next node. Before we delete a node, we need to store next pointer of the node



Implementation:
Functions other than removeDuplicates() are just to create a linked linked list and test removeDuplicates().

C++

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/* 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 beginging 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; 
    
  
/* Drier 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

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C

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/* 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 beginging 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;
    }
  
/* Drier 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;
}

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Java

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// Java program to remove duplicates from a sorted linked list
class 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==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();
     }
  
     /* Drier 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 */    

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Python 3

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# Python3 program to remove duplicate
# nodes from a sorted linked list 
  
# Node class 
class 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 
    # occurence 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 Code 
llist = 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.

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C#

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// C# program to remove duplicates
// from a sorted linked list
using 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 */

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Output:

 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 number of nodes in the given linked list.

Recursive Approach :

C++

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/* 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 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 the beginging 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 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

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C

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/* 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 beginging 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 */   

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Java

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// Java Program to remove duplicates
// from a sorted linked list 
class GFG
{
/* Link list node */
static class Node 
    int data; 
    Node next; 
}; 
  
// The function removes duplicates
// from a sorted list 
static 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 beginging
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; 
    
  
/* 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

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output

Linked list before duplicate removal  11 11 11 13 13 20 
Linked list after duplicate removal 11 13 20 

Related Article :
Remove all occurrences of duplicates from a sorted Linked List

References:
cslibrary.stanford.edu/105/LinkedListProblems.pdf



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