PHP - Insert a new node at the start of the Circular Doubly Linked List

In this method, a new node is inserted at the start of the circular doubly linked list. For example - if the given List is 10->20->30 and a new element 100 is added at the start, the List becomes 100->10->20->30.

Inserting a new node at the start of the circular doubly linked list is very easy. First, a new node with given element is created. It is then added at the start of the list by linking the head node and last node to the new node.

Circular Doubly Linked List - Add Node At Start

The function push_front is created for this purpose. It is a 6-step process.

public function push_front($newElement) {
  
  //1. allocate node
  $newNode = new Node();
  
  //2. assign data element
  $newNode->data = $newElement;
  
  //3. assign null to the next and prev
  //   of the new node
  $newNode->next = null; 
  $newNode->prev = null;

  //4. Check the list is empty or not,
  //   if empty make the new node as head 
  if($this->head == null) {
    $this->head = $newNode;
    $newNode->next = $this->head;
    $newNode->prev = $this->head;
  } else {
    
    //5. Else, traverse to the last node
    $temp = new Node();
    $temp = $this->head;
    while($temp->next !== $this->head) {
      $temp = $temp->next;
    }
    
    //6. Adjust the links
    $temp->next = $newNode;
    $newNode->prev = $temp;
    $newNode->next = $this->head;
    $this->head->prev = $newNode;
    $this->head = $newNode;
  }    
}

The below is a complete program that uses above discussed concept to insert new node at the start 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 start of the list
  public function push_front($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->prev = $temp;
      $newNode->next = $this->head;
      $this->head->prev = $newNode;
      $this->head = $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 start of the list.
$MyList->push_front(10);
$MyList->push_front(20);
$MyList->push_front(30);
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 30 20 10