class Node {

	public int data;
	public Node next;
	public Node prev;
	
	public Node(int data){
		this.data=data;
	}
	
	public String toString(){
		return ""+data;
	}
}
/**
 * Recursive Pair swapping in a linked list
 * @author Prateek
 */
 class PairSwapRecursive {
	/**
	 * Recursive Subroutine to swap pairs of nodes of linked list
	 * @return new Head of the linked list
	 */
	public Node pairSwap(Node head){
		Node curr=head;
		Node front=null;
	
		if(curr!=null && curr.next!=null){
			front=curr.next;
			curr.next = pairSwap(front.next);
			front.next=curr;
		}
		return (front!=null) ? front : curr;
	}

	public static void main(String[] args) {
		PairSwapRecursive obj = new PairSwapRecursive() ;

		Node root = new Node(1) ;
		root.next =new Node(2) ;
		root.next.next =new Node(3) ;
		root.next.next.next =new Node(4) ;
		root.next.next.next.next =new Node(5) ;
		//root.next.next.next.next.next =new Node(6) ;
		
		Node head=obj.pairSwap(root);
		obj.display(head);
	}
	
	/**
	 * Display the nodes of the linked list
	 */
	public void display(Node root)	{
		Node tempNode;
		tempNode=root;
		while(tempNode!=null){
			System.out.print(tempNode.data+"-->");
			tempNode=tempNode.next;
		}
		System.out.print("null");
		System.out.println();
	}
}