1
/**
2
 * Definition for singly-linked list.
3
 * struct ListNode {
4
 *     int val;
5
 *     ListNode *next;
6
 *     ListNode() : val(0), next(nullptr) {}
7
 *     ListNode(int x) : val(x), next(nullptr) {}
8
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
9
 * };
10
 */
11
class Solution {
12
 public:
13
  ListNode *reverseList(ListNode *head) {
14
    if (head == NULL || head->next == NULL) {
15
      return head;
16
    }
17
    ListNode *s = reverseList(head->next);
18
    ListNode *t = head->next;
19
    t->next = head;
20
    head->next = NULL;
21
    return s;
22
  }
23
};