/**
 * 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)
            return head;
        ListNode* l1=NULL;
        ListNode* l2=head;
        while(l2)
        {
            ListNode* tmp=l2->next;
            l2->next=l1;
            l1=l2;
            l2=tmp;
        }
        return l1;
    }
};