Input:

->->

Output:

->->

 /**

  * Definition for singly-linked list.

  * struct ListNode {

  *     int val;

  *     ListNode *next;

  *     ListNode(int x) : val(x), next(NULL) {}

  * };

  */

 class Solution {

 public:

     int help(ListNode* head) {

         if (head->next == NULL) head->val++;

         else head->val += help(head->next);

         if (head->val < ) return ;

         head->val = ;

         return ;

     }

     ListNode* plusOne(ListNode* head) {

         int credit = help(head);

         if (!credit) return head;

         ListNode *res = new ListNode();

         res->next = head;

         return res;

     }

 };