below is my code, could someone help me fix it. I ran test cases [3, 9, 8] + [5, 7, 8, 9] = [8, 6, 7, 0, 1] and [0] + [0] = [0]. both passed.

```
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
boolean overTen = false;
ListNode result = l1;
ListNode previous = l1;
if (l1 == null) return l2;
if (l2 == null) return l1;
while (l1 != null) {
if (l2 != null) {
int tmpVal = l1.val + l2.val + (overTen ? 1 : 0);
l1.val = tmpVal %10;
overTen = tmpVal / 10 > 0;
}
else if (overTen) {
overTen = (l1.val + 1 ) / 10 > 0;
l1.val = (l1.val + 1) %10;
}
else {
return result;
}
previous = l1;
l1 = l1.next;
l2 = l2.next;
}
previous.next = l2;
while (l2 != null) {
if (overTen) {
int tmp = (overTen ? 1 : 0) + l2.val;
l2.val = tmp % 10;
overTen = tmp / 10 > 0;
previous = l2;
l2 = l2.next;
}
}
if (overTen) {
ListNode last = new ListNode(1);
previous.next = last;
}
return result;
}
```