PHP - Insert a new node at a given position in the Circular Doubly Linked List

In this method, a new element is inserted at the specified position in the circular doubly linked list. For example - if the given list is 10->20->30 and a new element 100 is added at position 2, the list becomes 10->100->20->30.

First, create two nodes: 1. newNode with given element and 2. temp to traverse through the list. After that count the number of elements in the list to check whether the insertion position is valid or not (It must lie in the range of [1, n + 1], where n is number of elements in the list). If the insertion position is valid and equal to 1 then make the newNode as head and adjust links accordingly. If the insertion position is valid and greater than 1 then traverse to the specified position and insert the newNode and adjust links accordingly.

Circular Doubly Linked List - Add Node At Start

The function push_at is created for this purpose. It is a 5-step process.

public function push_at($newElement, $position) {     

  //1. allocate node to new element and 
  //   create a temp node to traverse the list 
  $newNode = new Node(); 
  $newNode->data = $newElement;
  $newNode->next = null;
  $temp = $this->head;
  $NoOfElements = 0;

  //2. Find the number of elements in the list
  if($temp != null) {
    $NoOfElements++;
    $temp = $temp->next;
  }
  while($temp != $this->head) {
    $NoOfElements++;
    $temp = $temp->next;
  }

  //3. check if the insertion position is valid
  if($position < 1 || $position > ($NoOfElements+1)) {
    echo "\nInvalid position.";
  } else if ($position == 1) {
  
  //4. if the position is 1, make next of the
  //   new node as head and new node as head
    if($this->head == null) {
      $this->head = $newNode;
      $this->head->next = $this->head;
      $this->head->prev = $this->head;
    } else {
      while($temp->next != $this->head) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->prev = $temp;
      $newNode->next = $this->head;
      $this->head->prev = $newNode;
      $this->head = $newNode;
    }
  } else {

   //5. Else, traverse to the node previous to
   //   the given position, make newNode next 
   //   as temp next and temp next as newNode.
    $temp = $this->head;
    for($i = 1; $i < $position-1; $i++) 
      $temp = $temp->next;
    $newNode->next = $temp->next;
    $newNode->next->prev = $newNode;
    $newNode->prev = $temp;
    $temp->next = $newNode;  
  }
}

The below is a complete program that uses above discussed concept to insert a new node at given position in 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 end of the list
  public function push_back($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->next = $this->head;
      $newNode->prev = $temp;
      $this->head->prev = $newNode;
    }    
  }

  //Inserts a new element at the given position 
  public function push_at($newElement, $position) {     
    $newNode = new Node(); 
    $newNode->data = $newElement;
    $newNode->next = null;
    $temp = $this->head;
    $NoOfElements = 0;

    if($temp != null) {
      $NoOfElements++;
      $temp = $temp->next;
    }
    while($temp != $this->head) {
      $NoOfElements++;
      $temp = $temp->next;
    }

    if($position < 1 || $position > ($NoOfElements+1)) {
      echo "\nInvalid position.";
    } else if ($position == 1) {
      if($this->head == null) {
        $this->head = $newNode;
        $this->head->next = $this->head;
        $this->head->prev = $this->head;
      } else {
        while($temp->next != $this->head) {
          $temp = $temp->next;
        }
        $temp->next = $newNode;
        $newNode->prev = $temp;
        $newNode->next = $this->head;
        $this->head->prev = $newNode;
        $this->head = $newNode;
      }
    } else {

      $temp = $this->head;
      for($i = 1; $i < $position-1; $i++) 
        $temp = $temp->next;
      $newNode->next = $temp->next;
      $newNode->next->prev = $newNode;
      $newNode->prev = $temp;
      $temp->next = $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 end of the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->PrintList();

//Insert an element at position 2
$MyList->push_at(100, 2);
$MyList->PrintList();

//Insert an element at position 1
$MyList->push_at(200, 1);
$MyList->PrintList(); 
?>

The above code will give the following output:

The list contains: 10 20 30 
The list contains: 10 100 20 30 
The list contains: 200 10 100 20 30