java - Remove duplicates from an unsorted linked list

Question

import java.util.*;
/*
 *  Remove duplicates from an unsorted linked list
 */
public class LinkedListNode {  
    public int data;  
    public LinkedListNode next;  

    public LinkedListNode(int data) {  
        this.data = data;    
    }  
}

public class Task {
    public static void deleteDups(LinkedListNode head){
      Hashtable<Integer, Boolean> table=new Hashtable<Integer, Boolean>();
      LinkedListNode previous=null;
      //nth node is not null
      while(head!=null){
        //have duplicate
            if(table.containsKey(head.data)){
                            //skip duplicate
                previous.next=head.next;
            }else{
            //put the element into hashtable
            table.put(head.data,true);
            //move to the next element
            previous=head;
            }
      //iterate
      head=head.next;
      }
   }
   public static void main (String args[]){
       LinkedList<Integer> list=new LinkedList<Integer>();
       list.addLast(1);
       list.addLast(2);
       list.addLast(3);
       list.addLast(3);
       list.addLast(3);
       list.addLast(4);
       list.addLast(4);
       System.out.println(list);
       LinkedListNode head=new LinkedListNode(list.getFirst());
       Task.deleteDups(head);
       System.out.println(list);
   }
}

The result: [1, 2, 3, 3, 3, 4, 4] [1, 2, 3, 3, 3, 4, 4]

It does not eliminate the duplicates.

Why doesn't the method work?

Answer 1

Iterate through the linked list, adding each element to a hash table. When we discover a duplicate element, we remove the element and continue iterating. We can do this all in one pass since we are using a linked list.

The following solution takes O(n) time, n is the number of element in the linked list.

public static void deleteDups (LinkedListNode n){
  Hashtable table = new Hashtable();
  LinkedListNode previous = null;
  while(n!=null){
      if(table.containsKey(n.data)){
          previous.next = n.next;
      } else {
          table.put(n.data, true);
          previous = n;
      }
      n = n.next;
  }
}