class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        while(!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;
    }
};