PHP - Search an element in the Circular Doubly Linked List

Searching an element in a circular doubly linked list requires creating a temp node pointing to the head of the list. Along with this, two more variables are required to track search and track index of the current node. If the temp node is not null at the start, then traverse th list to check if current node value matches with the search value. If it matches then update search tracker variable and stop traversing the list, else keep on traversing the list. If the temp node is empty at the start, then the list contains no item.

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

public function SearchElement($searchValue) {
  
  //1. create a temp node pointing to head
  $temp = new Node();
  $temp = $this->head;

  //2. create two variables: found - to track
  //   search, idx - to track current index
  $found = 0;
  $i = 0;

  //3. if the temp node is not null check the
  //   node value with searchValue, if found 
  //   update variables and break the loop, else
  //   continue searching till temp node is not head 
  if($temp != null) {
    while(true) {
      $i++;
      if($temp->data == $searchValue) {
        $found++;
        break;
      }
      $temp = $temp->next;
      if($temp == $this->head) {break;}
    }
    if ($found == 1) {
      echo $searchValue." is found at index = ".$i.".\n";
    } else {
      echo $searchValue." is not found in the list.\n";
    }
  } else {
    
    //4. If the temp node is null at the start, 
    //   the list is empty
    echo "The list is empty.\n";
  }
}

The below is a complete program that uses above discussed concept to search an element in a given 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;
    }    
  }

  //Search an element in the list
  public function SearchElement($searchValue) {
    $temp = new Node();
    $temp = $this->head;
    $found = 0;
    $i = 0;

    if($temp != null) {
      while(true) {
        $i++;
        if($temp->data == $searchValue) {
          $found++;
          break;
        }
        $temp = $temp->next;
        if($temp == $this->head) {break;}
      }
      if ($found == 1) {
        echo $searchValue." is found at index = ".$i.".\n";
      } else {
        echo $searchValue." is not found in the list.\n";
      }
    } else {
      echo "The list is empty.\n";
    }
  }  

  //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);

//traverse to display the content of the list.
$MyList->PrintList();

//search for element in the list
$MyList->SearchElement(10);
$MyList->SearchElement(15);
$MyList->SearchElement(20);
?>

The above code will give the following output:

The list contains: 10 20 30 
10 is found at index = 1.
15 is not found in the list.
20 is found at index = 2.