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C# Program For Removing Duplicates From A Sorted Linked List

  • Last Updated : 14 Dec, 2021

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(). 

C#




// C# program to remove duplicates
// from a sorted linked list
using System;
      
public class LinkedList
{
    // Head of list
    Node head; 
  
    // 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;
            }
   
            // Advance if no deletion
            else 
                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 

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 the number of nodes in the given linked list.

Recursive Approach :  

C#




// C# Program to remove duplicates
// from a sorted linked list 
using System;
      
class GFG
{
    // Link list node 
    public class Node 
    
        public int data; 
        public 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 
       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; 
        
    
  
    // 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);                                     
  
        Console.Write("Linked list before"
                      " duplicate removal "); 
        printList(head); 
  
        // Remove duplicates from linked list 
        head = removeDuplicates(head); 
  
        Console.Write("Linked list after"
                      " duplicate removal ");     
        printList(head);             
    }
}
// This code is contributed by PrinciRaj1992

Output:

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

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#




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

Output:

List before removal of duplicates
11 11 11 13 13 20 
List after removal of elements
11 13 20 

 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#




// C# program for the above approach
using 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(
                "Linked list after duplicate removal  ");
        removeDuplicates(head);
    }
}
// This code is contributed by rag2127

Output:

Linked list before duplicate removal 11 11 11 13 13 20

Time Complexity: O(Number of Nodes)

Space Complexity: O(Number of Nodes)

Please refer complete article on Remove duplicates from a sorted linked list for more details!


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