```
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode result = l1;
ListNode current = null;
int additive = 0;
while (l1 != null) {
l1.val = l1.val + (l2 == null ? 0 : l2.val) + additive;
additive = l1.val / 10;
l1.val = l1.val % 10;
if (current != null)
current.next = l1;
current = l1;
l1 = l1.next;
if (l2 != null) l2 = l2.next;
//swap if l2 longer
if (l1 == null && l2 != null) {
l1 = l2;
l2 = null;
}
}
if (additive > 0)
current.next = new ListNode(additive);
return result;
}
}
```