gistfile1.php[345]

<?php 

class Link {
	public $value;
	public $next;
	public $previous;
	
	public function __construct($value) {
		$this->value = $value;
		$this->next = null;
		$this->previous = null;
	}
	
	public function displayLink() {
		echo $this->value.' ';
	}
}

class DoublyLinkedList {
	private $first;
	private $last;
	
	public function __construct() {
		$this->first = null;
		$this->last = null;
	}
	
	public function isEmpty() {
		return empty($this->first);
	}
	
	public function displayForward() {
		if ($this->isEmpty()) {
			echo 'the list is null';
			
			return null;
		}

		$current = $this->first;
		while ($current != null) {
			$current->displayLink();
			
			$current = $current->next;
		}
	}
	
	public function displayBackward() {
		if ($this->isEmpty()) {
			echo 'the list is null';
			
			return null;
		}

		$current = $this->last;
		while ($current !== null) {
			$current->displayLink();
			
			$current = $current->previous;
		}
	}
	
	public function insertFirst($value) {
		$link = new Link($value);
		
		if ($this->isEmpty()) {
			$this->first = $this->last = $link;

			return ;
		}

		$this->first->previous = $link;
		$link->next = $this->first;
		$this->first = $link;
	}
	
	public function insertLast($value) {
		$link = new Link($value);

		if ($this->isEmpty()) {
			$this->first = $this->last = $link;

			return ;
		}

		$link->previous = $this->last;
		$this->last->next = $link;
		$this->last = $link;
	}
	
	public function insertAfter($value, $insertValue) {
		if ($this->isEmpty()) {
			echo 'the list is null';

			return null;
		}

		$current = $this->first;
		while ($current !== null && $current->value != $value) {
			$current = $current->next;
		}
		
		if ($current == null) {
			echo 'not found the '.$this->value.' link';

			return ;
		}
		
		$link = new Link($insertValue);
		
		if ($current == $this->last) {
			$this->last = $link;	
		} else {
			$link->next = $current->next;
			$current->next->previous = $link;
		}
		
		$link->previous = $current;
		$current->next = $link;
	}
	
	public function deleteFirst() {
	
	}
	
	public function deleteLast() {
	
	}
	
	public function deleteKey() {
			
	}
}

$list = new DoublyLinkedList();
$list->insertFirst(11);
$list->insertFirst(22);
$list->displayForward();
echo "\n";
$list->displayBackward();
echo "\n";

$list = new DoublyLinkedList();
$list->insertLast(44);
$list->insertLast(33);
$list->displayForward();
echo "\n";
$list->displayBackward();
echo "\n";
$list->insertAfter(44, 55);
$list->insertAfter(33, 66);
$list->displayForward();
?>