储存字符串的PHP单链表

<meta charset="utf-8" />
<?
class Node{
    public $data;
    public $next;

    function __construct($data, $next = null){
        $this->data = $data;
        $this->next = $next;
    }    
}

class LinkedList{
    private $head; //单链表的头结点，不存储数据

 function __construct(){//单链表的构造方法
 	//头结点的数据为"傀儡", 不代表 任何数据
    $this->head = new Node("dummy 傀儡"); 
    $this->first = null;
    }

 function isEmpty(){ 
        return ($this->head->next == null);
    }
	
/*	orderInsert($data) 方法， 
 *  按给定字符串 $data 的大小, 将其安插到适当的位置，	
 *  以保证单链表中字符串的存储，始终是有序的。 
 */
 function orderInsert($data){
	$p = new Node($data);	
    if($this->isEmpty()){
        $this->head->next = $p;
    }
    else{
    $q = $this->head;
    while($q->next != NULL && strcmp($data, $q->next->data)> 0 ){
    $q = $q->next;
		}
        $p->next = $q->next; 
		$q->next = $p;
    }     	
 }
 
 function insertLast($data){//将字符串插到单链表的尾部
    $p = new Node($data);
	
    if($this->isEmpty()){
        $this->head->next = $p;
    }
    else{
        $q = $this->head->next;
        while($q->next != NULL)
            $q = $q->next;
        $q->next = $p;  
    }     
}

    function find($value){//查询是否有给定的字符串
        $q = $this->head->next;
        while($q->next != null){
        if(strcmp($q->data,$value)==0){
				break;
			}
            $q = $q->next;    
        }
		if ($q->data == $value)
        return $q; 
		else
		return new Node($value." 没有找到！");
    }

    function traversal(){//遍历单链表
		if(!$this->isEmpty()){
        $p=$this->head->next;
        echo $p->data." ";
        while($p->next != null){
			$p=$p->next;
			echo $p->data." ";
			}			
    	}else
		echo "链表为空！";
	}
} 

$ll = new LinkedList();
$city =array("Wuhan", "Beijing", "Shanghai", "Tianjin", "Changsha", "Kunming", "Gongyi","Tokyo","New York","Ottawa","Moskow","Edmonton","Glasgow","Thunder Bay","New Delhi"); 

for ($i=0;$i<count($city);$i++)
$ll->orderInsert($city[$i]);
$ll->traversal();
echo "<br>".$ll->find("Gongyi")->data."<br>";
echo $ll->find("Baoding")->data."<br>";
?>