PHP Data Structures - Circular Doubly Linked List Other Related Topics

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



In this method, a node at the specified position in the circular doubly linked list is deleted. For example - if the given list is 10->20->30 and the 2nd node is deleted, the list becomes 10->20.

First, create two nodes temp and nodeToDelete to traverse through the list and track the node to delete respectively. After that count the number of elements in the list to check whether the specified position is valid or not (It must lie in the range of [1, n], where n is number of elements in the list). If the specified valid position is 1 and head is the only element in the list, then make the head as null. If the specified valid position is 1 and list contains more than one elements, then make next of head as new head and adjust links accordingly. If the specified valid position is greater than 1 then traverse to the node previous to the given position and delete the given node and adjust links accordingly.

Circular Doubly Linked List - Delete Node

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

public function pop_at($position) {     

  //1. create two nodes - temp and nodeToDelete 
  //   to traverse and track the node to delete  
  $nodeToDelete = $this->head;
  $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 specified position is valid
  if($position < 1 || $position > $NoOfElements) {
    echo "\nInvalid position.";
  } else if ($position == 1) {
  
  //4. if the position is 1 and head is the only element
  //   in the list, then make it null, else make next
  //   of head as new head and adjust links accordingly
    if($this->head->next == $this->head) {
      $this->head = null;
    } else {
      while($temp->next != $this->head)
        $temp = $temp->next;
      $this->head = $this->head->next;
      $temp->next = $this->head; 
      $this->head->prev = $temp;
      $nodeToDelete = null; 
    }
  } else {

   //5. Else, traverse to the node previous to
   //   the given position and delete the given 
   //   node and adjust links accordingly 
    $temp = $this->head;
    for($i = 1; $i < $position-1; $i++) 
      $temp = $temp->next;
    $nodeToDelete = $temp->next;
    $temp->next = $temp->next->next;
    $temp->next->prev = $temp;
    $nodeToDelete = null;  
  }
} 

The below is a complete program that uses above discussed concept to delete a node at a 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;
    }    
  }

  //Delete an element at the given position
  public function pop_at($position) {     
    $nodeToDelete = $this->head;
    $temp = $this->head;
    $NoOfElements = 0;

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

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

//Delete an element at position 2
$MyList->pop_at(2);
$MyList->PrintList();

//Delete an element at position 1
$MyList->pop_at(1);
$MyList->PrintList(); 
?>

The above code will give the following output:

The list contains: 10 20 30
The list contains: 10 30
The list contains: 30 

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