php 实现双向链表

 /**
  * 链表元素结点类
  */
 class Node {
     public $pre = NULL; // 前驱
     public $next = NULL; // 后继
     public $data = NULL; // 结点值
     public function __Construct($data) {
         $this->data = $data;
     }
 }

 /**
  * 双向链表类
  */
 class DoubleLink {
     private $head; // 头指针
     private $tail; // 尾指针
     private $len; // 链表长度

     /**
      * 初始化链表
      */
     public function __Construct()
     {
         $newNode = $newNode = new Node(null);
         $this->tail = $this->head = $newNode;
         $this->len = 0;
     }

     /**
      * 添加结点
      * @param $data 要添加的结点
      * @param $search 添加的位置
      */
     public function addNode($data, $search = null) {
         $newNode = new Node($data);
         $tmp = $this->searchNode($search);
         if ($tmp !== null) {
             $newNode->pre = $tmp->pre;
             $newNode->next = $tmp;
             $tmp->pre = $newNode;
             $newNode->pre->next = $newNode;
         } else {
             $newNode->pre = $this->tail;
             $this->tail->next = $newNode;
             $this->tail = $newNode;
         }
         $this->len++;
     }

     /**
      * 删除指定结点
      * @param $search
      */
     public function delNode($search) {
         $tmp = $this->searchNode($search);
         if(null !== $tmp){
             if ($tmp->next !== null) {
                 $tmp->pre->next = $tmp->next;
                 $tmp->next->pre = $tmp->pre;
             } else {
                 $tmp->pre->next = null;
             }
             unset($tmp);
             $this->len--;
         }
     }

     /**
      * 修改指定结点的值
      * @param $search
      * @param $data
      */
     public function setNode($search, $data){
         $tmp = $this->searchNode($search);
         if(null !== $tmp){
             $tmp->data = $data;
         }
     }

     /**
      * 查找结点
      * @param $search 要查找的结点元素值
      * @return $tmp 查找到的结点元素
      */
     public function searchNode($search) {
         $tmp = $this->head;
         while ( $tmp->next !== null ) {
             $tmp = $tmp->next;
             if ($tmp->data === $search) {
                 return $tmp;
             }
         }
         return null;
     }

     /**
      * 读取链表全部结点
      */
     public function show() {
         $tmp = $this->head;
         while ( $tmp->next !== null ) {
             $tmp = $tmp->next;
             echo $tmp->data;
         }
         echo "<br/>";
         $tmp = $this->tail;
         while ( $tmp->pre !== null ) {
             echo $tmp->data;
             $tmp = $tmp->pre;
         }
     }
 }

 $myList = new DoubleLink();
 $myList->addNode("A");
 $myList->addNode("B");
 $myList->addNode("C");
 $myList->addNode("D");
 $myList->addNode("E");
 $myList->addNode('F','C');
 $myList->delNode('F');
 $myList->setNode("B",'G');
 $myList->show();