[2018-Interview] Add Two Numbers

Original question: https://leetcode.com/problems/add-two-numbers/description/

My answer:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null) {
            return l2;
        } else if (l2 == null) {
            return l1;
        }
        int addOne = (l1.val + l2.val) / 10;
        int newVal = (l1.val + l2.val) % 10;
        ListNode root = new ListNode(newVal);
        l1 = l1.next;
        l2 = l2.next;
        ListNode currentNode = root;
        while (l1 != null && l2 != null) {
            newVal = (l1.val + l2.val + addOne) % 10;
            addOne = (l1.val + l2.val + addOne) / 10;
            ListNode newNode = new ListNode(newVal);
            currentNode.next = newNode;
            currentNode = currentNode.next;
            l1 = l1.next;
            l2 = l2.next;
        }
        ListNode remainNode = l1 == null? l2 : l1;
        while (addOne != 0) {
            int currentVal = 0;
            if (remainNode != null) {
                currentVal = remainNode.val;
            }
            newVal = (currentVal + addOne) % 10;
            addOne = (currentVal + addOne) / 10;
            ListNode newNode = new ListNode(newVal);
            currentNode.next = newNode;
            currentNode = currentNode.next;
            remainNode = remainNode != null? remainNode.next : null;
        }
        if (remainNode != null) {
            currentNode.next = remainNode;
        }
        return root;
    }
}