1 /** 2  * Definition for singly-linked list. 3  * public class ListNode { 4  *     int val; 5  *     ListNode next; 6  *     ListNode(int x) { val = x; } 7  * } 8  */ 9 public class Solution {10     public ListNode plusOne(ListNode head) {11         if (helper(head) == 0) {12             return head;13         } 14         ListNode result = new ListNode(1);15         result.next = head;16         return result;17     }18     19     private int helper(ListNode head) {20         if (head == null) {21             return 1;22         }23         24         int current = helper(head.next);25         int nextLevel = 0;26         nextLevel = (head.val + current) / 10;27         head.val = (head.val + current) % 10;28         return nextLevel;29     }30 }