Plus One Linked List

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode plusOne(ListNode head) {
        if (helper(head) == 0) {
            return head;
        } 
        ListNode result = new ListNode(1);
        result.next = head;
        return result;
    }
    
    private int helper(ListNode head) {
        if (head == null) {
            return 1;
        }
        
        int current = helper(head.next);
        int nextLevel = 0;
        nextLevel = (head.val + current) / 10;
        head.val = (head.val + current) % 10;
        return nextLevel;
    }
}