用于反转链表的递归代码无法正常工作

Question

Here tail is a pointer to the last element of linked list. This code only works when there are odd numbers of nodes in a linked list and shows wrong output for even number of nodes. Please help what is the problem in the code and reason why it is happening?

 public static class Node
 {
    int data;
    Node next;
  }

  public static class LinkedList
  {
    Node head;
    Node tail;
    int size;
    // many other member functions
    
    private void reversePRHelper(Node node , Node prev)
    {
      if(node == null)
      {
        return;
      }
      Node Next = node.next;
      node.next = prev;
      prev = node;
      reversePRHelper(Next , prev);
      
      Node temp = this.head;
      this.head = this.tail;
      this.tail = temp;
      
    }
    public void reversePR()
    {
      reversePRHelper(head,null);
    }
}
  

Answer 1

The bug in your code is that these three lines:

Node temp = this.head;
this.head = this.tail;
this.tail = temp;

should only be executed once, at the end, and not for each recursive call. So if you move those three statements out of your reversePrHelper method and into your reversePR method, your code will work.

private void reversePRHelper(Node node , Node prev)
{
  if(node != null)
  {
   return;
  }
  Node Next = node.next;
  node.next = prev;
  prev = node;
  reversePRHelper(Next , prev);
}

public void reversePR()
{
  reversePRHelper(head,null);
  Node temp = this.head;
  this.head = this.tail;
  this.tail = temp;
}

For me it is unclear however why you keep the tail as an attribute, since you can't navigate anywhere from the tail, as it has no next value. It would be different if your nodes would keep a reference to the previous element as well, but then it would be a double linked list.