PHP - Circular Singly Linked List

A circular singly 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 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 last node points to HEAD. A circular singly linked list can be visualized as a chain of nodes, where every node points to the next node. Along with this, next of the last node is linked to the head node.

Implementation of Circular Singly 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;
}

class LinkedList {
  public $head; 

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

Create a Circular Singly Linked List

Let us create a simple circular 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;
//linking with head node
$MyList->head = $first;
//linking next of the node with head
$first->next = $MyList->head;

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

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

Traverse a Circular Singly Linked List

A circular singly linked list can be traversed from any node of the list using a temp node. Keep on moving the temp node to the next one and displaying its content. Stop the traversal, after reaching the starting node.

<?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(true) {
        echo $temp->data." ";
        $temp = $temp->next;
        if($temp == $this->head)
          break;        
      }
      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;
//linking with head node
$MyList->head = $first;
//linking next of the node with head
$first->next = $MyList->head;

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

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

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

The above code will give the following output:

The list contains: 10 20 30