PHP - Delete odd nodes of the Linked List

Deleting odd nodes of a linked list requires traverse through the list and deleting odd nodes one by one. If the list is not null then release the head and make next of head as new head. If the new head is not null then create two nodes - evenNode and oddNode. Make evenNode to the first even node of list and oddNode to second odd node of the list (3^rd node in the original list). If both are not null, delete the oddNode and adjust links. Move both nodes to next set of even-odd nodes. Repeat the process till the any or both nodes become null.

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

public function deleteOddNodes() {
  if($this->head != null) {
    
    //1. if head is not null, make next of head as
    //   new head and delete previous head
    $temp = $this->head;
    $this->head = $this->head->next;
    $temp = null;
    if($this->head != null) { 
      
      //2. if the new head is not null create 
      //   nodes - evenNode and oddNode
      $evenNode = $this->head;
      $oddNode = $this->head->next; 

      while($evenNode != null && $oddNode != null) {
        
        //3. while evenNode and oddNode are not null
        //   make next of evenNode as next of oddNode 
        //   and free oddNode  
        $evenNode->next = $oddNode->next;
        $oddNode = null;

        //4. and make evenNode as next of evenNode
        $evenNode = $evenNode->next;
        
        //5. Update evenNode and oddNode
        if($evenNode != null)
          $oddNode = $evenNode->next;
      }
    }
  }
}

The below is a complete program that uses above discussed concept of deleting odd nodes of a linked list.

<?php
//node structure
class Node {
  public $data;
  public $next;
}

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;
    $newNode->next = null; 
    if($this->head == null) {
      $this->head = $newNode;
    } else {
      $temp = new Node();
      $temp = $this->head;
      while($temp->next != null) {
        $temp = $temp->next;
      }
      $temp->next = $newNode;
    }    
  }

  //delete odd nodes of the list
  public function deleteOddNodes() {
    if($this->head != null) {
      $temp = $this->head;
      $this->head = $this->head->next;
      $temp = null;
      if($this->head != null) { 
        $evenNode = $this->head;
        $oddNode = $this->head->next; 
        while($evenNode != null && $oddNode != null) {
          $evenNode->next = $oddNode->next;
          $oddNode = null;
          $evenNode = $evenNode->next;
          if($evenNode != null)
            $oddNode = $evenNode->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 odd nodes of the list
$MyList->deleteOddNodes();

echo "After deleting odd 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 odd nodes.
The list contains: 20 40