Java Program For Removing Duplicates From An Unsorted Linked List

Last Updated : 28 Jun, 2023

Given an unsorted Linked List, the task is to remove duplicates from the list.

Examples:

Input: linked_list = 12 -> 11 -> 12 -> 21 -> 41 -> 43 -> 21
Output: 12 -> 11 -> 21 -> 41 -> 43
Explanation: Second occurrence of 12 and 21 are removed.

Input: linked_list = 12 -> 11 -> 12 -> 21 -> 41 -> 43 -> 21
Output: 12 -> 11 -> 21 -> 41 -> 43

Naive Approach to Remove Duplicates from an Unsorted Linked List:

The most simple approach to solve this, is to check each node for duplicate in the Linked List one by one.

Below is the Implementation of the above approach:

Java

// Java program to remove duplicates from unsorted
// linked list
 
class LinkedList {
 
    static Node head;
 
    static class Node {
 
        int data;
        Node next;
 
        Node(int d)
        {
            data = d;
            next = null;
        }
    }
 
    /* Function to remove duplicates from an
       unsorted linked list */
    void remove_duplicates()
    {
        Node ptr1 = null, ptr2 = null, dup = null;
        ptr1 = head;
 
        /* Pick elements one by one */
        while (ptr1 != null && ptr1.next != null) {
            ptr2 = ptr1;
 
            /* Compare the picked element with rest
                of the elements */
            while (ptr2.next != null) {
 
                /* If duplicate then delete it */
                if (ptr1.data == ptr2.next.data) {
 
                    /* sequence of steps is important here
                     */
                    ptr2.next = ptr2.next.next;
                    System.gc();
                }
                else /* This is tricky */ {
                    ptr2 = ptr2.next;
                }
            }
            ptr1 = ptr1.next;
        }
    }
 
    void printList(Node node)
    {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }
 
    public static void main(String[] args)
    {
        LinkedList list = new LinkedList();
        list.head = new Node(10);
        list.head.next = new Node(12);
        list.head.next.next = new Node(11);
        list.head.next.next.next = new Node(11);
        list.head.next.next.next.next = new Node(12);
        list.head.next.next.next.next.next = new Node(11);
        list.head.next.next.next.next.next.next
            = new Node(10);
 
        System.out.println(
            "Linked List before removing duplicates : ");
        list.printList(head);
 
        list.remove_duplicates();
        System.out.println("\n");
        System.out.println(
            "Linked List after removing duplicates : ");
        list.printList(head);
    }
}
// This code has been contributed by Mayank Jaiswal

Output

Linked List before removing duplicates : 
10 12 11 11 12 11 10 

Linked List after removing duplicates : 
10 12 11

Time Complexity: O(N²)
Auxiliary Space: O(1)

Remove duplicates from an Unsorted Linked List using Sorting:

Follow the below steps to Implement the idea:

Sort the elements using Merge Sort for Linked Lists.
Remove duplicates in linear time using the algorithm for removing duplicates in sorted Linked List.

Below is the implementation for above approach:

Java

import java.io.*;
 
// structure of a node in the linked list
class Node {
    int data;
    Node next;
 
    Node(int d)
    {
        data = d;
        next = null;
    }
}
 
// class for the linked list
class LinkedList {
    Node head;
 
    // function to insert a node in the linked list
    public void push(int new_data)
    {
        Node new_node = new Node(new_data);
        new_node.next = head;
        head = new_node;
    }
 
    // function to sort the linked list
    public void sortList()
    {
        // pointer to traverse the linked list
        Node current = head;
        Node index = null;
 
        // loop to traverse the linked list
        while (current != null) {
            // loop to compare current node with all other
            // nodes
            index = current.next;
            while (index != null) {
                // checking for duplicate values
                if (current.data == index.data) {
                    // deleting the duplicate node
                    current.next = index.next;
                    index = current.next;
                }
                else {
                    index = index.next;
                }
            }
            current = current.next;
        }
    }
 
    // function to display the linked list
    public void printList()
    {
        Node node = head;
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
        System.out.println();
    }
}
 
// main class
class Main {
    public static void main(String[] args)
    {
        LinkedList ll = new LinkedList();
        ll.push(20);
        ll.push(13);
        ll.push(13);
        ll.push(11);
        ll.push(11);
        ll.push(11);
 
        System.out.println(
            "Linked List before removing duplicates : ");
        ll.printList();
 
        ll.sortList();
 
        System.out.println(
            "Linked List after removing duplicates : ");
        ll.printList();
    }
}

Output

Linked List before removing duplicates : 
11 11 11 13 13 20 
Linked List after removing duplicates : 
11 13 20

Time Complexity: O(N log N)
Auxiliary Space: O(1)

Remove duplicates from an Unsorted Linked List using Hashing:

The idea for this approach is based on the following observations:

Traverse the link list from head to end.

For every newly encountered element, check whether if it is in the hash table:

if yes, remove it

otherwise put it in the hash table.

At the end, the Hash table will contain only the unique elements.

Below is the implementation of the above approach:

Java

// Java program to remove duplicates
// from unsorted linkedlist
 
import java.util.HashSet;
 
public class removeDuplicates {
    static class node {
        int val;
        node next;
 
        public node(int val) { this.val = val; }
    }
 
    /* Function to remove duplicates from a
       unsorted linked list */
    static void removeDuplicate(node head)
    {
        // Hash to store seen values
        HashSet<Integer> hs = new HashSet<>();
 
        /* Pick elements one by one */
        node current = head;
        node prev = null;
        while (current != null) {
            int curval = current.val;
 
            // If current value is seen before
            if (hs.contains(curval)) {
                prev.next = current.next;
            }
            else {
                hs.add(curval);
                prev = current;
            }
            current = current.next;
        }
    }
 
    /* Function to print nodes in a given linked list */
    static void printList(node head)
    {
        while (head != null) {
            System.out.print(head.val + " ");
            head = head.next;
        }
    }
 
    public static void main(String[] args)
    {
        /* The constructed linked list is:
         10->12->11->11->12->11->10*/
        node start = new node(10);
        start.next = new node(12);
        start.next.next = new node(11);
        start.next.next.next = new node(11);
        start.next.next.next.next = new node(12);
        start.next.next.next.next.next = new node(11);
        start.next.next.next.next.next.next = new node(10);
 
        System.out.println(
            "Linked list before removing duplicates :");
        printList(start);
 
        removeDuplicate(start);
 
        System.out.println(
            "\nLinked list after removing duplicates :");
        printList(start);
    }
}
 
// This code is contributed by Rishabh Mahrsee

Output

Linked list before removing duplicates :
10 12 11 11 12 11 10 
Linked list after removing duplicates :
10 12 11

Time Complexity: O(N), on average (assuming that hash table access time is O(1) on average).
Auxiliary Space: O(N), As extra space is used to store the elements in the stack.