PHP - Circular Doubly Linked List Traversal

Traversing through a circular doubly linked list is very easy. It requires creating a temp node pointing to the head of the list. If the temp node is not null, display its content and move to the next node using temp next. Repeat the process till the temp node reaches the starting node (head of the list). If the temp node is empty at the start, then the list contains no item.

A circular doubly linked list can be traversed from any node as the starting point. Traverse the list till reaches the same node.

The function PrintList is created for this purpose. It is a 3-step process.

public function PrintList() {
  
  //1. create a temp node pointing to head
  $temp = new Node();
  $temp = $this->head;
  
  //2. if the temp node is not null continue 
  //   displaying the content and move to the 
  //   next node till the temp becomes head
  if($temp != null) {
    echo "The list contains: ";
    while(true) {
      echo $temp->data." ";
      $temp = $temp->next;
      if($temp == $this->head)
        break;        
    }
    echo "\n";
  } else {
    
    //3. If the temp node is null at the start, 
    //   the list is empty
    echo "The list is empty.\n";
  }
}

The below is a complete program that uses above discussed concept to traverse through the circular doubly linked list and displaying its content.

<?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;
    }    
  }

  //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 three elements at the end of the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);

//traverse to display the content of the list.
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30