/**
 * 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) {
        //迭代
        /*if(head==NULL)return NULL;
        ListNode* first=NULL;
        ListNode* second=head;
        while(second!=NULL){
            ListNode* tmp=second->next;
            second->next=first;
            first=second;
            second=tmp;
        }
        return first;*/
        //递归
        if(head==NULL||head->next==NULL)return head;
        ListNode* p=reverseList(head->next);
        head->next->next=head;
        head->next=NULL;
        return p;
    }
};