206. 反转链表

反转一个单链表。

示例:

输入: 1->2->3->4->5->NULL
输出: 5->4->3->2->1->NULL

/**
 * 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 *ans = reverseList(head->next);
          head->next->next = head;
          head->next = NULL;
          return ans;
    }
};