Reverse Singly Linked List Java [duplicate]

Question

Can someone tell me why my code dosent work? I want to reverse a single linked list in java: This is the method (that doesnt work correctly)

public void reverseList(){     Node before = null;     Node tmp = head;     Node next = tmp.next;     while(tmp != null){       if(next == null)          return;       tmp.next = before;       before = tmp;       tmp = next;       next = next.next;     } } 

And this is the Node class:

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

On input 4->3->2->1 I got output 4. I debugged it and it sets pointers correctly but still I dont get why it outputs only 4.

Answer 1

Node next = tmp.next; while(tmp != null){ 

So what happens when tmp == null?

You almost got it, though.

Node before = null; Node tmp = head; while (tmp != null) {     Node next = tmp.next;     tmp.next = before;     before = tmp;     tmp = next; } head = before; 

Or in nicer (?) naming:

Node reversedPart = null; Node current = head; while (current != null) {     Node next = current.next;     current.next = reversedPart;     reversedPart = current;     current = next; } head = reversedPart; 

ASCII art:

        <__<__<__ __ : reversedPart    : head                  (__)__ __ __ head :   current:      >  >  > 

Answer 2

public Node<E> reverseList(Node<E> node) {     if (node == null || node.next == null) {         return node;     }     Node<E> currentNode = node;     Node<E> previousNode = null;     Node<E> nextNode = null;      while (currentNode != null) {         nextNode = currentNode.next;         currentNode.next = previousNode;         previousNode = currentNode;         currentNode = nextNode;     }     return previousNode; }