PHP - Reverse the Circular Doubly Linked List

While working with a circular doubly linked list, sometimes it is required to reverse it. Reversing a List produces following result: if the given List is 10->20->30->40->50, after reversing the List the List becomes 50->40->30->20->10.

Reversing a List requires creating three nodes, considering that the list is not empty, which are as follows: tempNode pointing to head, prevNode pointing to head and curNode pointing to next of head. Then make next and previous of prevNode as itself to make the first node as last node of the reversed list. After that, while the curNode is not head adjust links (unlink curNode and link it to the reversed list from front and modify curNode and prevNode to the next element in the list). At last, make the prevNode (last node) as head.

The function reverseList is created for this purpose. It is a 4-step process.

public function reverseList() {
  //1. If head is not null create three nodes
  //   prevNode - pointing to head,
  //   tempNode - pointing to head,
  //   curNode - pointing to next of head
  if($this->head != null) {
    $prevNode = $this->head;
    $tempNode = $this->head;
    $curNode = $this->head->next;
    
    //2. assign next and previous of prevNode 
    //   as itself to make the first node as last 
    //   node of the reversed list
    $prevNode->next = $prevNode;
    $prevNode->prev = $prevNode;
    
    while($curNode != $this->head) {
      //3. While the curNode is not head adjust links 
      //   (unlink curNode and link it to the reversed list 
      //   from front and modify curNode and prevNode)  
      $tempNode = $curNode->next;

      $curNode->next = $prevNode;
      $prevNode->prev = $curNode;
      $this->head->next = $curNode;
      $curNode->prev = $this->head;

      $prevNode = $curNode;
      $curNode = $tempNode;
    }

    //4. Make prevNode (last node) as head
    $this->head = $prevNode;
  }
}

The below is a complete program that uses above discussed concept to reverse a given circular doubly linked list.

<?php
//node structure
class Node {
  public $data;
  public $next;
  public $prev;
}

class LinkedList {
  public $head;

  public function __construct(){
    $this->head = null;
  }
  
  //Add new element at the end of the list
  public function push_back($newElement) {
    $newNode = new Node();
    $newNode->data = $newElement;
    $newNode->next = null; 
    $newNode->prev = null;
    if($this->head == null) {
      $this->head = $newNode;
      $newNode->next = $this->head;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next !== $this->head) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->next = $this->head;
      $newNode->prev = $temp;
      $this->head->prev = $newNode;
    }    
  }

  //reverse the list
  public function reverseList() {
    if($this->head != null) {
      $prevNode = $this->head;
      $tempNode = $this->head;
      $curNode = $this->head->next;
      
      $prevNode->next = $prevNode;
      $prevNode->prev = $prevNode;
      
      while($curNode != $this->head) { 
        $tempNode = $curNode->next;

        $curNode->next = $prevNode;
        $prevNode->prev = $curNode;
        $this->head->next = $curNode;
        $curNode->prev = $this->head;

        $prevNode = $curNode;
        $curNode = $tempNode;
      }

      $this->head = $prevNode;
    }
  } 

  //display the content of the list
  public function PrintList() {
    $temp = new Node();
    $temp = $this->head;
    if($temp != null) {
      echo "The list contains: ";
      while(true) {
        echo $temp->data." ";
        $temp = $temp->next;
        if($temp == $this->head)
          break;        
      }
      echo "\n";
    } else {
      echo "The list is empty.\n";
    }
  }    
};

// test the code  
$MyList = new LinkedList();

//Add five elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(40);
$MyList->push_back(50);

//Display the content of the list.
$MyList->PrintList();

//Reversing the list.
$MyList->reverseList();
  
//Display the content of the list.
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30 40 50 
The list contains: 50 40 30 20 10