Doubly Linked List - Basic

DDL.java

// same without static variable
// https://gist.github.com/anil477/9ddf5717431f54a8d2ce53ea13b90c0c
class Node
{
    int data;
    Node next;
    Node previous;
    
    public Node(int data)
    {
        next      = null;
        previous  = null;
        this.data = data;
    }
}

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 newNode = new Node(data);
        if(size==0)
        {
            head = newNode;
            tail = newNode;
        }
        else
        {
            newNode.next = head;
            head.previous = newNode;
            head = newNode;
        }
        size++;
        return newNode;
    }

    public Node addAtEnd(int data)
    {
        System.out.println("Adding Node " + data + " at the End");
        
        Node newNode = new Node(data);
        if(size==0)
        {
            head = newNode;
            tail = newNode;
        }
        else
        {
            tail.next = newNode;
            newNode.previous = tail;
            tail     = newNode;
        }
        size++;
        return newNode;
    }

    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);
            Node newNode = new Node(data);

            //store the next node of prevNode
            Node nextNode = prevNode.next;

            //make new node next points to prevNode
            newNode.next = nextNode;

            //make prevNode next points to new Node
            prevNode.next = newNode;

            //make nextNode previous points to new node
            nextNode.previous = newNode;

            //make  new Node previous points to prevNode
            newNode.previous = prevNode;
            
            size++;
            
            return newNode;
        }
    }

    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
        {
            int x = tail.data;
            tail = tail.previous;
            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(index-1!=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());
    }
}