/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* reverseList(ListNode* head) {
if (!head) return head;
auto a = head, b = a->next;
a->next = NULL;
while (b)
{
auto tmp = b->next;
b->next = a;
a = b;
b = tmp;
}
return a;
}
};