PHP 逆转单链表

<?php
    #逆转单链表
    class Node {
        public $data = null;
        public $next = null;
    }

    #递归版本
    #思想是一直递归到倒数第二个非空节点，并将其next->next指向自己，将自己的next指向null
    #为了获得逆转后的头结点，在最后一个非空节点，即cnode->next == null时，将节点返回
    function reverse_list_r($node) {
        if ($node->next == null) {
            return $node;
        } else {
            $head = reverse_list_r($node->next);
            $node->next->next = $node;
            $node->next = null;
            return $head;
        }
    }

    #非递归版本
    #思想是需要保存下三个节点，分别是cnode,next(cnode->next),next2(cnode->next->next)
    #每次要执行的操作只有将next->next指向cnode，然后依次将这三个节点后移，直到next == null
    function reverse_list($head) {
        $cnode = $head;
        $next = $head->next;
        $head->next = null;
        while ($next != null) {
            $next2 = $next->next;
            $next->next = $cnode; 
            $cnode = $next;
            $next = $next2;
        }

        return $cnode;
    }

    #遍历单链表
    function traverse($head) {
        $cnode = $head;
        while ($cnode != null) {
            echo $cnode->data . " ";
            $cnode = $cnode->next;
        }
        echo "<br>";
    }

    $head = new Node();
    $n1 = new Node();
    $n2 = new Node();
    $n3 = new Node();
    $head->data = 0;
    $n1->data = 1;
    $n2->data = 2;
    $n3->data = 3;
    $head->next = $n1;
    $n1->next = $n2;
    $n2->next = $n3;
    $n3->next = null;
    traverse($head);

    $rhead = reverse_list_r($head);
    traverse($rhead);

    $rrhead = reverse_list($rhead);
    traverse($rrhead);
?>

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