/
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;
}

};