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:
1->2->3

Output:
1->2->4

思路：递归。

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     int help(ListNode* head) {
12         if (head->next == NULL) head->val++;
13         else head->val += help(head->next);
14         if (head->val < 10) return 0;
15         head->val = 0;
16         return 1;
17     }
18     ListNode* plusOne(ListNode* head) {
19         int credit = help(head);
20         if (!credit) return head;
21         ListNode *res = new ListNode(1);
22         res->next = head;
23         return res;
24     }
25 };