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// @Title: 反转链表 (Reverse Linked List)
// @Author: 15816537946@163.com
// @Date: 2022-03-21 14:42:41
// @Runtime: 8 ms
// @Memory: 20.8 MB
/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {

    /**
     * @param ListNode $head
     * @return ListNode
     */
     function reverseList($head)
    {
        if ($head->next === null) return $head;
        $last = $this->reverseList($head->next);
        $head->next->next = $head;
        $head->next = null;
        return $last;
    }
}