Plus One Linked List -- LeetCode

Given a non-negative number represented as a singly linked list of digits, plus one to the number.

The digits are stored such that the most significant digit is at the head of the list.

Example:

Input:
->->

Output:
->->

思路：递归。

 /**
  * 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;
     }
 };