样例

/**
 * 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;

        ListNode *a = head, *b = head->next;
        while(b)
        {
            ListNode *c  = b->next;
            b->next = a;
            a = b;
            b = c;
        }
        head -> next = NULL;
        return a;
    }
};