PHP 带哨兵双向队列

<?php
    #带哨兵的双链表实现

    #链表节点
    class Node {
        public $pre = null;
        public $next = null;
        public $key = null;
    }

    #查找链表值
    function search($list, $key) {
        $cnode = $list->next;
        while ($cnode !== $list && $cnode->key !== $key) {
            $cnode = $cnode->next;
        }

        if ($cnode === $list) {
            return false;
        } else {    
            return $cnode;
        }
    }

    #插入节点
    function insert($list, $new_node) {
        $list->next->pre = $new_node;
        $new_node->next = $list->next;
        $new_node->pre = $list;
        $list->next = $new_node;
    }

    #删除链表节点
    function delete($del_node) {
        $del_node->next->pre = $del_node->pre;
        $del_node->pre->next = $del_node->next;
        unset($del_node);
    }

    #遍历链表
    function traverse($list) {
        $cnode = $list->next;
        while ($cnode !== $list) {
            echo $cnode->key . " ";
            $cnode = $cnode->next;
        }
        echo "<br>";
    }

    $list = new Node(); #list节点是个哨兵
    $list->pre = $list;
    $list->next = $list;
    $head = new Node();
    $mid = new Node();
    $tail = new Node();
    $head->key = 0;
    $mid->key = 1;
    $tail->key = 2;
    $list->pre = $tail;
    $list->next = $head;
    $head->pre = $list;
    $head->next = $mid;
    $mid->pre = $head;
    $mid->next = $tail;
    $tail->pre = $mid;
    $tail->next = $list;
    traverse($list);
    $insert = new Node();
    $insert->key = 3;
    insert($list, $insert);
    traverse($list);
    delete($insert);
    traverse($list);
    print_r(search($list, 1)->key);
?>

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3 0 1 2 
0 1 2 
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