Java program to remove duplicate elements from a Doubly Linked List

In this program, we will create a doubly linked list and remove the duplicate, if present, by traversing through the list.

Original List:

Java program to remove duplicate elements from a Doubly Linked List

List after removing duplicates:

Java program to remove duplicate elements from a Doubly Linked List

In above list, node2 is repeated thrice, and node 3 is repeated twice. Current will point to head, and index will point to node next to current. Start traversing the list till a duplicate is found that is when current’s data is equal to index’s data. In above example, the first duplicate will be found at position 4. Assign index to another node temp. Connect index’s previous node with index’s next node. Delete temp which was pointing to duplicate node. This process will continue till all duplicates are removed.

Algorithm

Define a Node class which represents a node in the list. It will have three properties: data, previous which will point to the previous node and next which will point to the next node.
Define another class for creating a doubly linked list, and it has two nodes: head and tail. Initially, head and tail will point to null.
addNode() will add node to the list:
- It first checks whether the head is null, then it will insert the node as the head.
- Both head and tail will point to a newly added node.
- Head’s previous pointer will point to null and tail’s next pointer will point to null.
- If the head is not null, the new node will be inserted at the end of the list such that new node’s previous pointer will point to tail.
- The new node will become the new tail. Tail’s next pointer will point to null.

a. removeDuplicateNode() will remove duplicate nodes from the list.

Define a new node current which will initially point to head.
Node index will always point to node next to current.
Loop through the list until current points to null.
Check whether current?s data is equal to index’s data that means index is duplicate of current.
Node temp will point to index to store duplicate node.
The previous node to the index will point to next node to index.
Since, temp is pointing to index, which is a duplicate node, so set temp to null.

a. display() will show all the nodes present in the list.

Define a new node ‘current’ that will point to the head.
Print current.data till current points to null.
Current will point to the next node in the list in each iteration.

Program:

  public class RemoveDuplicate {      //Represent a node of the doubly linked list      class Node{          int data;          Node previous;          Node next;          public Node(int data) {              this.data = data;          }      }      //Represent the head and tail of the doubly linked list      Node head, tail = null;        //addNode() will add a node to the list      public void addNode(int data) {          //Create a new node          Node newNode = new Node(data);            //If list is empty          if(head == null) {              //Both head and tail will point to newNode              head = tail = newNode;              //head’s previous will point to null              head.previous = null;              //tail’s next will point to null, as it is the last node of the list              tail.next = null;          }          else {              //newNode will be added after tail such that tail’s next will point to newNode              tail.next = newNode;              //newNode’s previous will point to tail              newNode.previous = tail;              //newNode will become new tail              tail = newNode;              //As it is last node, tail’s next will point to null              tail.next = null;          }      }      //removeDuplicateNode() will remove duplicate nodes from the list      public void removeDuplicateNode() {          //Node current will point to head          Node current, index, temp;            //Checks whether list is empty          if(head == null) {              return;          }          else {              //Initially, current will point to head node              for(current = head; current != null; current = current.next) {                  //index will point to node next to current                  for(index = current.next; index != null; index = index.next) {                      if(current.data == index.data) {                          //Store the duplicate node in temp                          temp = index;                          //index’s previous node will point to node next to index thus, removes the duplicate node                          index.previous.next = index.next;                          if(index.next != null)                              index.next.previous = index.previous;                          //Delete duplicate node by making temp to null                          temp = null;                      }                  }              }          }      }        //display() will print out the nodes of the list      public void display() {          //Node current will point to head          Node current = head;          if(head == null) {              System.out.println(“List is empty”);              return;          }          while(current != null) {              //Prints each node by incrementing the pointer.                System.out.print(current.data + ” “);              current = current.next;          }          System.out.println();      }        public static void main(String[] args) {            RemoveDuplicate dList = new RemoveDuplicate();          //Add nodes to the list          dList.addNode(1);          dList.addNode(2);          dList.addNode(3);          dList.addNode(2);          dList.addNode(2);          dList.addNode(4);          dList.addNode(5);          dList.addNode(3);            System.out.println(“Originals list: “);          dList.display();            //Removes duplicate nodes          dList.removeDuplicateNode();            System.out.println(“List after removing duplicates: “);          dList.display();      }  }  

Output:

Originals list:   1 2 3 2 2 4 5 3   List after removing duplicates:   1 2 3 4 5

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Java program to remove duplicate elements from a Doubly Linked List