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× PHP Data Structures - Doubly Linked List Other Related Topics


In this method, a new node is inserted at the start of the 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 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 to the new node.

Doubly Linked List - Add Node At Start

The function push_front is created for this purpose. It is a 5-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;
  } else {
    
    //5. Adjust the links and make the new
    //   node as head
    $this->head->prev = $newNode;
    $newNode->next = $this->head;
    $this->head = $newNode;
  }    
}

The below is a complete program that uses above discussed concept to insert new node at the start of the 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;
    } else {
      $this->head->prev = $newNode;
      $newNode->next = $this->head;
      $this->head = $newNode;
    }    
  }

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

// 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