/**
 * 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; //如果链表个数小于等于1，则输出头结点

        auto p=head, q=head->next;
        while(q)
        {
            auto o=q->next;
            q->next=p;
            p=q;
            q=o;
        }
        head->next=NULL; //此时head为尾结点，应该指向空
        return p;      //最后新的头结点为p
    }

};