public ListNode reverseList(ListNode head) {

         if(head == null) return null;

         ListNode res = new ListNode(0);

         res.next = head;

         ListNode cur = head;

         while(cur.next != null){

             ListNode tmp = cur.next;

             cur.next = tmp.next;

             tmp.next = res.next;

             res.next = tmp;

         }

         return res.next;

 }

 public ListNode reverse(ListNode head){

         ListNode prev = null;

         while(head != null){

             ListNode next = head.next;

             head.next = prev;

             prev = head;

             head = next;

         }

         return prev;

     }

 ListNode* reverseList(ListNode* head) {

         if (!head || !(head -> next)) return head;

         ListNode* node = reverseList(head -> next);

         head -> next -> next = head;

         head -> next = NULL;

         return node;

 }