PHP - Doubly Linked List

A doubly linked list is a linear data structure, in which the elements are stored in the form of a node. Each node contains three sub-elements. A data part that stores the value of the element, the previous part that stores the link to the previous node, and the next part that stores the link to the next node as shown in the below image:

The first node also known as HEAD is always used as a reference to traverse the list. The previous of head node and next of last node points to NULL. A doubly linked list can be visualized as a chain of nodes, where every node points to previous and next node.

Implementation of Doubly Linked List

Representation:

In PHP, doubly linked list can be represented as a class and a Node as a separate class. The LinkedList class contains a reference of Node class type.

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

class LinkedList {
  public $head; 

  //constructor to create an empty LinkedList
  public function __construct(){
    $this->head = null;
  }
};

Create a Doubly Linked List

Let us create a simple doubly linked list which contains three data nodes.

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

class LinkedList {
  public $head; 

  //constructor to create an empty LinkedList
  public function __construct(){
    $this->head = null;
  } 
};

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

//Add first node.
$first = new Node();
$first->data = 10;
$first->next = null;
$first->prev = null;
//linking with head node
$MyList->head = $first;

//Add second node.
$second = new Node();
$second->data = 20;
$second->next = null;
//linking with first node
$second->prev = $first;
$first->next = $second;

//Add third node.
$third = new Node();
$third->data = 30;
$third->next = null;
//linking with second node
$third->prev = $second;
$second->next = $third;
?>

Traverse a Doubly Linked List

A doubly linked list can be traversed using a temp node. Keep on moving the temp node to the next one and displaying its content. At the end of the list, the temp node will become NULL.

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

class LinkedList {
  public $head;

  //constructor to create an empty LinkedList
  public function __construct(){
    $this->head = null;
  }

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

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

//Add first node.
$first = new Node();
$first->data = 10;
$first->next = null;
$first->prev = null;
//linking with head node
$MyList->head = $first;

//Add second node.
$second = new Node();
$second->data = 20;
$second->next = null;
//linking with first node
$second->prev = $first;
$first->next = $second;

//Add third node.
$third = new Node();
$third->data = 30;
$third->next = null;
//linking with second node
$third->prev = $second;
$second->next = $third;

//print the content of list
$MyList->PrintList();   
?>

The above code will give the following output:

The list contains: 10 20 30