Re: I got it working, but have trouble reversing the list

From: D. Beckham (lekhanh88_at_earthlink.net)
Date: 05/31/04 

Date: Mon, 31 May 2004 21:57:57 GMT

Thanks. I fixed the link list and was able to print out all the elements.
However, I spend 2 hrs trying to reverse the list. I made many changes and
always get a null pointer exception when the ReverseList method is called.
Would you please help me with an algorithm to reverse the list?

public class TestNode
{
 private static DLLNode head = null;
 private static DLLNode tail = null;
 //private static int count = 0;
 private static DLLNode [] curNode = new DLLNode [2];

 public static void main(String[] args)
 {
  DLLNode hNode = new DLLNode(null, null, null); //create head node
  DLLNode tNode = new DLLNode("Three", null, hNode); //create tail node
  DLLNode cNode = new DLLNode("One"); //insert cNode after head
  DLLNode cNode1 = new DLLNode("Two"); //insert cNode1 after cNode

  //Create doubly-link nodes
  hNode.setNext(cNode);
  cNode.setPrev(hNode);
  cNode.setNext(cNode1);
  cNode1.setPrev(cNode);
  cNode1.setNext(tNode);
  tNode.setPrev(cNode1);

  //assign newly created objects to global variables
  head = hNode;
  tail = tNode;
  curNode[0] = cNode;
  curNode[1] = cNode1;

  //print nodes
  DLLNode p = head.getNext();
  printList(p);

  //reverse list
  printList(reverseList(head));
 }

 public static void printList (DLLNode head) //print list
 {
  if(head.getNext() == null)
  {
   System.out.println("List is empty");
   return;
  }
  while(head != null)
  {
   System.out.println(head.getElement());
   head = head.getNext();
  }
 }

/////////////////////I have trouble with this particular
method/////////////////////////////////////////////
 public static DLLNode reverseList (DLLNode head) //returns the head of
reversed list
 {
  DLLNode temp = null;
  DLLNode p = head;

  while(p != null){
   temp = p.getNext();
   p.setElement(p.getNext());
   p.setPrev(temp);
  }
  return p;
 }

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