PHP - Delete odd nodes of the Circular Singly Linked List

Deleting odd nodes of a circular singly linked list requires traverse through the list and deleting odd nodes one by one. It involves the following process. If the list contains only head then make the head null. If the list contains more than one element then delete the head and adjust the link of last element with the new head. If next of head is not head then, create three nodes - evenNode pointing to head, oddNode pointing to next of head and temp to store last even node. Then delete oddNode and update evenNode and oddNode to next set of odd-even nodes and continue the process till any of the node reaches head. After that if evenNode reaches head, make next of temp as head else make next of evenNode as head.

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

public function deleteOddNodes() {
  //1. if head is the only element element in 
  //   list make the head as null  
  if($this->head != null && $this->head->next == $this->head) {
    $this->head = null;
  } else if($this->head != null) {

    //2. if the list contains more than one element
    //   delete the head and adjust the link of
    //   last element with the new head       
    $temp = $this->head;
    while($temp->next != $this->head) {
      $temp = $temp->next;
    }
    $temp->next = $this->head->next;
    $this->head = null;
    $this->head = $temp->next;

    //3. create evenNode node - pointing to head
    //   oddNode node - pointing to next of head
    //   temp node - to store last even node
    if($this->head != null && $this->head->next != $this->head) {

      $evenNode = $this->head;
      $oddNode = $this->head->next; 
      while(true) {
        
        //4. delete odd node and update evenNode and 
        //   oddNode to next set of odd-even nodes update
        //   temp node to latest evenNode node continue
        //   the process till any of the node reaches head
        $temp = $evenNode;
        $evenNode->next = $oddNode->next;
        $oddNode = null;
        $evenNode = $evenNode->next;
        $oddNode = $evenNode->next;
        if($evenNode == $this->head || $oddNode == $this->head)
          break;
      }
      
      //5. if evenNode reaches head, make next of temp
      //   as head else make next of evenNode as head
      if($evenNode == $this->head)
        $temp->next = $this->head;
      else
        $evenNode->next = $this->head;
    }
  } 
}

The below is a complete program that uses above discussed concept of deleting odd nodes of a circular singly 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;
      $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;
    }    
  }

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

      if($this->head != null && $this->head->next != $this->head) {

        $evenNode = $this->head;
        $oddNode = $this->head->next; 
        while(true) {
          $temp = $evenNode;
          $evenNode->next = $oddNode->next;
          $oddNode = null;
          $evenNode = $evenNode->next;
          $oddNode = $evenNode->next;
          if($evenNode == $this->head || $oddNode == $this->head)
            break;
        }

        if($evenNode == $this->head)
          $temp->next = $this->head;
        else
          $evenNode->next = $this->head;
      }
    } 
  } 

  //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 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