/**
* 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 || !head -> next) return head;
auto p = head,q = p -> next;
while(q){
auto o = q -> next;
q -> next = p;
p = q;
q = o;
}
head -> next = NULL;
return p;
}
};