Doubly Linked List Complete Implementation

In this article we will see what is doubly linked list, how it is different from other linked list and how to implement it.

Earlier we have seen what is Singly Linked List and Circular Linked List and How to implement it. In a way you say that it’s an extension of singly linked list. I would suggest that if you do not about Linked list, first read “Singly Linked List”

Let’s see the difference between singly and doubly linked list.

Singly Linked List vs Doubly Linked List

Singly Linked List Doubly Linked List
Easy Implement Not easy
Less memory More Memory
Can traverse only in forward direction Traverse in both direction, back and froth

Doubly Linked List

So as we can see the in doubly linked list each node has two references, next and prev which makes it possible to traverse in both directions.

Let’s see how the Node structure will look like

Doubly Linked List - Node

class Node{
   int data;
   Node next;
   Node previous;
   public Node(int data){
      this.data = data;
      next = null;
      previous = null;
   }
}

 

Operations:

NOTE: we are two references here, head and tail. Head points the start of the linked list and tail points to the last node of the linked list.

Add at the Start : Add a node the begin­ning of the linked list. Its O(1). If size is 0 then make the new node as head and tail else put the at the start, change the head and do not change the tail.

Add at the End : Add a node at the end of the linked list. its O(1) since we have tail reference. If size is 0 then make the new node as head and tail else put node at the end of the list using tail reference and make the new node as tail.

Delete at the Start : Delete a node from begin­ning of the linked list and make the head points to the 2nd node in the list. Its O(1).

Get Size: returns the size of the linked list.

Get Ele­ment at Index : Return the ele­ment at spe­cific index, if index is greater than the size then return –1. its O(n) in worst case.

Print: Prints the entire linked list. O(n).

Com­plete Code:


public class DoublyLinkedList {
int size =0;
Node head = null;
Node tail = null;
public Node addAtStart(int data){
System.out.println("Adding Node " + data + " at the start");
Node n = new Node(data);
if(size==0){
head = n;
tail = n;
}else{
n.next = head;
head.previous = n;
head = n;
}
size++;
return n;
}
public Node addAtEnd(int data){
System.out.println("Adding Node " + data + " at the End");
Node n = new Node(data);
if(size==0){
head = n;
tail = n;
}else{
tail.next = n;
n.previous = tail;
tail =n;
}
size++;
return n;
}
public Node addAfter(int data, Node prevNode){
if(prevNode==null){
System.out.println("Node after which new node to be added cannot be null");
return null;
}else if(prevNode==tail){//check if it a last node
return addAtEnd(data);
}else{
System.out.println("Adding node after "+ prevNode.data);
//create a new node
Node n = new Node(data);
//store the next node of prevNode
Node nextNode = prevNode.next;
//make new node next points to prevNode
n.next = nextNode;
//make prevNode next points to new Node
prevNode.next = n;
//make nextNode previous points to new node
nextNode.previous = n;
//make new Node previous points to prevNode
n.previous = prevNode;
size++;
return n;
}
}
public void deleteFromStart(){
if(size==0){
System.out.println("\nList is Empty");
}else{
System.out.println("\ndeleting node " + head.data + " from start");
head = head.next;
size;
}
}
public void deleteFromEnd(){
if(size==0){
System.out.println("\nList is Empty");
}else if(size==1){
deleteFromStart();
}else{
//store the 2nd last node
int x = tail.data;
Node prevTail = tail.previous;
//detach the last node
tail = prevTail;
tail.next=null;
System.out.println("\ndeleting node " + x + " from end");
size;
}
}
public int elementAt(int index){
if(index>size){
return 1;
}
Node n = head;
while(index1!=0){
n=n.next;
index;
}
return n.data;
}
//get Size
public int getSize(){
return size;
}
public void print(){
Node temp = head;
System.out.print("Doubly Linked List: ");
while(temp!=null){
System.out.print(" " + temp.data);
temp = temp.next;
}
System.out.println();
}
public static void main(String[] args) {
DoublyLinkedList d = new DoublyLinkedList();
Node x = d.addAtStart(2);
d.addAtStart(1);
d.print();
d.addAtEnd(3);
d.print();
d.addAfter(4,x);
d.print();
d.deleteFromStart();
d.print();
System.out.println("Element at index 2: "+d.elementAt(2));
d.addAtStart(1);
d.print();
d.deleteFromEnd();
d.print();
System.out.println("Size of the Linked List: " + d.getSize());
}
}
class Node{
int data;
Node next;
Node previous;
public Node(int data){
this.data = data;
next = null;
previous = null;
}
}


Output:

Adding Node 2 at the start
Adding Node 1 at the start
Doubly Linked List:  1 2
Adding Node 3 at the End
Doubly Linked List:  1 2 3
Adding node after 2
Doubly Linked List:  1 2 4 3
deleting node 1 from start
Doubly Linked List:  2 4 3
Element at index 2: 4
Adding Node 1 at the start
Doubly Linked List:  1 2 4 3
deleting node 3 from end
Doubly Linked List:  1 2 4
Size of the Linked List: 3

6 thoughts on “Doubly Linked List Complete Implementation”

  1. Thank you for this precious resource. But I have two questions.
    1. Can you tell me why you declare the type of these method “Node” and you are returning n in these methods addAtStart, addAtEnd, addAfter?
    2. Don’t we declare these methods types void?

    Reply
    • Yes you can, it’s just a way u implement it. In the code we are just passing the integer and we are creating a node inside the function ,Sometimes you might need the reference of that node to be used so instead of traversing all the nodes, it’s good to have a reference back from the function.

      Reply
  2. Just wow for your logic! I just dirty my hands and at the end I realize what you did in your code that’s awesome! I will say “just wow”!

    Reply
  3. Thank for the tutorial. But I think return a Node type is a bug here. The “Node” type should not be exposed to outside, otherwise imagine:

    DoublyLinkedList d = new DoublyLinkedList();
    Node x = d.addAtStart(2);
    DoublyLinkedList d2 = new DoublyLinkedList();
    d2.addAfter(10, x);

    This will add a node into d, without change the members of d.
    Meanwhile, d2 keeps empty.

    In a word, I think the type Node must be hidden inside DoublyLinkedList…

    Reply
  4. Hi, Can anyone post a solution for this please :
    “Replace kth node from beginning with kth node from end in case of a doubly linked list”
    Either in java or in c.

    Reply

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