How to do a reversed linked list

Question

The problem is that in the inverted method the program never shows the first number. For example if the numbers to be reversed is 1 2 3 4, the out put is 3 2 1 0. there is no 4 and the 0 should not be there. please help.

import java.util.Scanner;


public class LinkTest
{
    public static void main(String [] args)
    {
        ListNode head = new ListNode();
        ListNode tail = head;
        int x;
        head.link = null;

        Scanner keyboard = new Scanner(System.in);

        System.out.println("Enter a list of integers ending in zero.");

        x = keyboard.nextInt();

        while(x != 0)
        {

            ListNode newOne = new ListNode();
            newOne.data = x;
            newOne.link = null;
            tail.link = newOne;
            tail = newOne;
            x = keyboard.nextInt();
        }

            printLinked(head);

            delRep(head);

            invert(head);

    }

    public static void printLinked(ListNode cursor)
    {

        while(cursor.link != null)
    {

        System.out.print(cursor.link.data + " ");
        cursor = cursor.link;
    }

    System.out.println();
    }

    public static void delRep(ListNode num)
    {   

        ListNode current = num.link;
        ListNode cursor = null;
        ListNode duplicate = null;

    while(current != null && current.link != null)
    {
        cursor = current;

        while(cursor.link != null)
        {
            if(current.data == cursor.link.data)
            {
                duplicate = cursor.link;
                cursor.link = cursor.link.link;
            }
            else
            {
                cursor = cursor.link;
            }
        }

        current = current.link;
    }


    System.out.println("Here is the list without repeated ");
    printLinked(num);


    }

public static void invert(ListNode head)
{
    ListNode previous = null;
    ListNode current = head;
    ListNode forward;

    while (current != null) 
    {
        forward = current.link;
        current.link = previous;
        previous = current;
        current = forward;
    }

        System.out.println("Here is the inverted list.");
        printLinked(previous);

    }
}

Answer 1

Your issue is that the head of your linked list is a dummy. For input 1 2 3 4 you create this:

0->1->2->3->4

When printing, you skip the first node.

But the invert method doesn't treat the head as a dummy. It reverts the list to this:

4->3->2->1->0

And then, since the printing method skips the head, the output you'll get is 3 2 1 0.

Here's the fix:

public static void invert(ListNode head)
{
    ListNode previous = null;
    ListNode current = head.link;
    ListNode forward;

    while (current != null)
    {
        forward = current.link;
        current.link = previous;
        previous = current;
        current = forward;
    }

    System.out.println("Here is the inverted list.");
    head.link = previous;
    printLinked(head);
}