/**
 * 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==NULL || head->next==NULL){          //单节点和空节点情况
            return head;
        }                   

        auto p = head;
        auto q = head->next;

        while(q){
            auto o = q->next;
            q->next = p;
            p = q;
            q = o;
        }
        head ->next = NULL;    //需要手动修改第一个指针域
        return p;
    }
};