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× PHP Data Structures - Doubly Linked List Other Related Topics


In this method, a node at the specified position in the 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, the specified position must be greater than equal to 1. If the specified position is 1 and head is not null, then make the head next as head and delete the previous head. Else, traverse to the node that is previous to the specified position. If the specified node and previous to the specified node are not null then adjust the link. In other case, the specified node will be already null. The below figure describes the process, if the deletion node is other than the head node.

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. check if the position is > 0
  if($position < 1) {
    echo "\nposition should be >= 1.";
  } else if ($position == 1 && $this->head != null) {
    
    //2. if the position is 1 and head is not null, make
    //   head next as head and delete previous head  
    $nodeToDelete = $this->head;
    $this->head = $this->head->next;
    $nodeToDelete = null;
    if ($this->head != null)
      $this->head->prev = null;
  } else {
    
    //3. Else, make a temp node and traverse to the 
    //   node previous to the position
    $temp = new Node();
    $temp = $this->head;
    for($i = 1; $i < $position-1; $i++) {
      if($temp != null) {
        $temp = $temp->next;
      }
    }
 
    //4. If the previous node and next of the previous  
    //   is not null, adjust links 
    if($temp != null && $temp->next != null) {
      $nodeToDelete = $temp->next;
      $temp->next = $temp->next->next;
      if($temp->next->next != null)
        $temp->next->next->prev = $temp->next;   
      $nodeToDelete = null; 
    } else {
      
      //5. Else the given node will be empty.
      echo "\nThe node is already null.";
    }       
  }
} 

The below is a complete program that uses above discussed concept to delete a node at a given position in the 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;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
      $newNode->prev = $temp;
    }    
  }

  //Delete an element at the given position
  public function pop_at($position) {     
    if($position < 1) {
      echo "\nposition should be >= 1.";
    } else if ($position == 1 && $this->head != null) {
      $nodeToDelete = $this->head;
      $this->head = $this->head->next;
      $nodeToDelete = null;
      if ($this->head != null)
        $this->head->prev = null;
    } else {
      $temp = new Node();
      $temp = $this->head;
      for($i = 1; $i < $position-1; $i++) {
        if($temp != null) {
          $temp = $temp->next;
        }
      }
      if($temp != null && $temp->next != null) {
        $nodeToDelete = $temp->next;
        $temp->next = $temp->next->next;
        if($temp->next->next != null)
          $temp->next->next->prev = $temp->next;   
        $nodeToDelete = null; 
      } else {
        echo "\nThe node is already null.";
      }       
    }
  } 

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

// 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