Two things to make the code simple:

- Whenever one of the two
*ListNode*is null, replace it with 0. - Keep the while loop going when at least one of the three conditions is met.

Let me know if there is something wrong. Thanks.

```
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
ListNode prev = new ListNode(0);
ListNode head = prev;
int carry = 0;
while (l1 != null || l2 != null || carry != 0) {
ListNode cur = new ListNode(0);
int sum = ((l2 == null) ? 0 : l2.val) + ((l1 == null) ? 0 : l1.val) + carry;
cur.val = sum % 10;
carry = sum / 10;
prev.next = cur;
prev = cur;
l1 = (l1 == null) ? l1 : l1.next;
l2 = (l2 == null) ? l2 : l2.next;
}
return head.next;
}
}
```