PHP - Linked List

A linked list is a linear data structure, in which the elements are stored in the form of a node. Each node contains two sub-elements. A data part that stores the value of the element and 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 last node points to NULL. Linked list can be visualized as a chain of nodes, where every node points to the next node.

Types of Linked List

The types of linked list are mentioned below:

Singly Linked List: can be traversed only in forward direction.
Doubly Linked List: can be traversed in forward and backward directions.
Circular Singly Linked List: Last element contains link to the first element as next.
Circular Doubly Linked List: Last element contains link to the first element as next and the first element contains link of the last element as previous.

Implementation of Singly Linked List

Representation:

In PHP, singly 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;
}

class LinkedList {
  public $head;

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

Create a Singly Linked List

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

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

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;
//linking with head node
$MyList->head = $first;

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

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

Traverse a Singly Linked List

A singly 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;
}

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;
//linking with head node
$MyList->head = $first;

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

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

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

The above code will give the following output:

The list contains: 10 20 30