PHP - Delete the first node of the Circular Doubly Linked List

In this method, the first node of the circular doubly linked list is deleted. For example - if the given list is 10->20->30->40 and the first node is deleted, the list becomes 20->30->40.

Deleting the first node of the circular doubly linked list is very easy. If the list contains one node, delete the node. If the list contains more than one node, then create two nodes - temp and firstNode both pointing to the head. Using temp node, traverse to the last node of the list. Make next of head as the head node and update all links. Finally, delete the first node.

Circular Singly Linked List - Delete First Node

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

public function pop_front() {
  if($this->head != null) {
    
    //1. the list contains one node, delete
    //   make the head null
    if($this->head->next == $this->head) {
      $this->head = null;
    } else {
      
      //2. if the list contains more than one node,
      //   create two nodes - temp and firstNode, both
      //   pointing to head node
      $temp = $this->head;
      $firstNode = $this->head;
      
      //3. using temp node, traverse to the last node
      while($temp->next != $this->head) {
        $temp = $temp->next;
      }
      
      //4. Make head as next of head, and 
      //   update links
      $this->head = $this->head->next;
      $this->head->prev = $temp;
      $temp->next = $this->head; 
      $firstNode = null; 
    }
  }
}

The below is a complete program that uses above discussed concept of deleting the first node of the 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;
    }    
  }

  //Delete first node of the list
  public function pop_front() {
    if($this->head != null) {
      if($this->head->next == $this->head) {
        $this->head = null;
      } else {
        $temp = $this->head;
        $firstNode = $this->head;
        while($temp->next != $this->head) {
          $temp = $temp->next;
        }
        $this->head = $this->head->next;
        $this->head->prev = $temp;
        $temp->next = $this->head; 
        $firstNode = null; 
      }
    }
  }

  //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);
$MyList->push_back(40);
$MyList->PrintList();

//Delete the first node
$MyList->pop_front();
$MyList->PrintList(); 
?>

The above code will give the following output:

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