PHP - Delete even nodes of the Doubly Linked List

Deleting even nodes of a doubly linked list requires traverse through the list and deleting even nodes one by one. It requires creating two nodes - oddNode and evenNode. If head is not null, make oddNode to first odd node of list and evenNode to first even node of the list. If both are not null, delete the evenNode and adjust links. Move both nodes to next set of odd-even nodes. Repeat the process till any or both nodes become null.

The function deleteEvenNodes is created for this purpose. It is a 4-step process.

public function deleteEvenNodes() {
  if($this->head != null) {
  
    //1. if head is not null create nodes -
    //   evenNode and oddNode
    $oddNode = $this->head;
    $evenNode = $this->head->next; 
     
    while($oddNode != null && $evenNode != null) {
      
      //2. while oddNode and evenNode are not null
      //   make next of oddNode as next of evenNode   
      //   and free evenNode
      $oddNode->next = $evenNode->next;
      $evenNode = null;

      //3. and make temp as oddNode and oddNode as 
      //   next of oddNode
      $temp = $oddNode;
      $oddNode = $oddNode->next;
      
      //4. Update prev link, oddNode and evenNode
      if($oddNode != null){
        $oddNode->prev = $temp;
        $evenNode = $oddNode->next;
      }
    }
  }
}

The below is a complete program that uses above discussed concept of deleting even nodes of a 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 even nodes of the list
  public function deleteEvenNodes() {
    if($this->head != null) {
      $oddNode = $this->head;
      $evenNode = $this->head->next;       
      while($oddNode != null && $evenNode != null) {
        $oddNode->next = $evenNode->next;
        $evenNode = null;

        $temp = $oddNode;
        $oddNode = $oddNode->next;
        if($oddNode != null){
          $oddNode->prev = $temp;
          $evenNode = $oddNode->next;
        }
      }
    }
  } 

  //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  
$MyList = new LinkedList();

//Add five elements in the list.
$MyList->push_back(10);
$MyList->push_back(20);
$MyList->push_back(30);
$MyList->push_back(40);
$MyList->push_back(50);

//Display the content of the list.
$MyList->PrintList();

//delete even nodes of the list
$MyList->deleteEvenNodes();

echo "After deleting even nodes.\n";
//Display the content of the list.
$MyList->PrintList();
?>

The above code will give the following output:

The list contains: 10 20 30 40 50 
After deleting even nodes.
The list contains: 10 30 50