class Solution {

 public:

     ListNode* reverseList(ListNode* head) {

         return reverseList_iteratively(head);

        //return reverseList_recursively(head);//递归方法leetcode超时

     }

 //迭代

     ListNode* reverseList_iteratively(ListNode* head)

     {

         ListNode* p = nullptr;

         ListNode* w = nullptr;

         while(head)

         {

             p = head->next;

             head->next = w;

             w = head;

             head = p;

         }

         return w;

     }

 //递归

      ListNode* reverseList_recursively(ListNode* head)

     {

         if(head==nullptr||head->next==nullptr)

         {

             return head;

         }

         ListNode* h = reverseList_recursively(head->next);

         head->next->next=head;//加入反转后的单链表尾部

         head = nullptr;

         return h;

     }

 };