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Java 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(). 

Java




// Java program to remove duplicates 
// from a sorted linked list
class LinkedList
{
    // Head of list
    Node head;  
   
    // 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();
     }
  
    // 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);
          
        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 


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 :  

Java




// 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 
       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; 
        
    
  
    // 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("Linked list after"
                           " duplicate removal ");     
        printList(head);             
    }
}  
// This code is contributed by Arnab Kundu


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:

Java




// Java program to remove duplicates
// from a sorted linked list
class 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 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);
          
         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 Arshita


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:

Java




// Java program for the above approach
import 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(
               "Linked list after duplicate removal  ");
        removeDuplicates(head);
    }
}
// This code is contributed by avanitrachhadiya2155


Output:

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