Java Program For Removing Duplicates From An Unsorted Linked List

Write a removeDuplicates() function that takes a list and deletes any duplicate nodes from the list. The list is not sorted. 
For example if the linked list is 12->11->12->21->41->43->21 then removeDuplicates() should convert the list to 12->11->21->41->43.

METHOD 1 (Using two loops):
This is the simple way where two loops are used. Outer loop is used to pick the elements one by one and the inner loop compares the picked element with the rest of the elements. 
Thanks to Gaurav Saxena for his help in writing this code.

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
                    dup = ptr2.next;
                    ptr2.next = ptr2.next.next;
                    System.gc();
                }
                else
                    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("");
        System.out.println(
               "Linked List after removing duplicates : ");
        list.printList(head);
    }
}
// This code is 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), where N is length of array.
Auxiliary Space: O(1)

METHOD 2 (Use Sorting):
In general, Merge Sort is the best-suited sorting algorithm for sorting linked lists efficiently. 
1) Sort the elements using Merge Sort. We will soon be writing a post about sorting a linked list. O(nLogn) 
2) Remove duplicates in linear time using the algorithm for removing duplicates in sorted Linked List. O(n) 
Please note that this method doesn’t preserve the original order of elements.
Time Complexity: O(nLogn)

METHOD 3 (Use Hashing):
We traverse the link list from head to end. For every newly encountered element, we check whether it is in the hash table: if yes, we remove it; otherwise we put it in the hash table.

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(
               "Linked 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

Thanks to bearwang for suggesting this method.
Time Complexity: O(n) on average (assuming that hash table access time is O(1) on average).
Auxiliary Space: O(n), considering the elements stored by the hashset.
Please refer complete article on Remove duplicates from an unsorted linked list for more details!