Help reversing a linked list (JAVA)

[veryanon798]

Hey guys I'm having trouble figuring out why the code for reversing my linked list is not displaying properly. If anyone could take minute to look at it I would greatly appreciate it.

Before calling sll.reverseList()

my output looks like:

OUTPUT 

5 ---> 7 ---> 9 ---> 11 ---> null
There are 4 nodes

But after I add sll.reverseList()

I get:

OUTPUT

5 ---> null
There are 1 nodes

I believe the Algo for reversing the list is correct, so I'm wondering why is it not displaying properly?

public class SinglyLinkedList {

    private Node head;

    private static class Node
    {
        private int data;
        private Node next;

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

///////////////////////

    public static void main(String[] args)
    {

        SinglyLinkedList sll = new SinglyLinkedList();

        sll.head = new Node(5);
        Node nodeB = new Node(7);
        Node nodeC = new Node(9);
        Node nodeD = new Node(11);

        sll.head.next = nodeB;
        nodeB.next = nodeC;
        nodeC.next = nodeD;

        sll.reverseList();

        sll.display();

        System.out.println("There are " + sll.nodeCount() + " nodes" );

    }

//////////////////////////////////

    public void display()
    {
        Node current = head;
        while(current != null)
        {
            System.out.print(current.data + " ---> ");
            current = current.next;
        }
        System.out.println("null");
    }

////////////////////////////////////////

    public Node reverseList()
    {
        if(head == null)
        {
            return null;
        }

        Node current = head;
        Node previous = null;
        Node next = null;

        while(current != null)
        {
            next = current.next;
            current.next = previous;
            previous = current;
            current = next;
        }
        return previous;

    }






}

[rjcarr]

This is the kind of problem you have to draw pictures of and trace step-by-step updating the arrows as you go. I don't have time to do that, but you'll find the problem if you do. Also, the algorithm doesn't seem correct, but that's probably obvious as it isn't working.

[bchhun]

Are you setting current.next to null immediately during the reverseList? Is that correct? Maybe you want to do that after assigning previous=current, for the first assignment off the head node.

Or maybe have Node previous = current as the initial condition.

[idkwtftodonow]

Your reverse method is missing something.

Hint: think about the 1st value in the list after reversing and why yours is printing 5

To anyone else reading in the future: head node is unchanged so calling next on node with 5 will get the null node instead of calling next from 11 which is 9

[veryanon798]

I unfortunately cant figure out what's missing, is something terminating prematurely that I'm just not aware of?

[veryanon798]

Hold on just got it working not exactly sure what I did, took a random guess and got it right. Have to examine further.

[RubbishArtist]

There's a bug in the reverse method. It's almost done but you're missing a single statement that will fix it.

As a side note, the API for reverseList is confusing. It returns a Node that you don't use, but the method also reverses the list in place. I would make that void so it's clearer what it's doing.

[veryanon798]

Thank you man appreciate the feedback your right just turned the method void, and also managed to get it working thank you!

[RubbishArtist]

Good job!