Share my in-place version, please let me know what you think! Thanks

```
public ListNode addLists(ListNode l1, ListNode l2) {
int carry = 0;
ListNode prev = null, ret = l1;
while (l1 != null || l2 != null) {
int sum = 0;
if (l1 != null) {
sum = l1.val + carry;
if (l2 != null) {
sum += l2.val;
l2 = l2.next;
}
carry = sum / 10;
l1.val = sum % 10;
prev = l1;
l1 = l1.next;
} else {
sum = l2.val + carry;
carry = sum / 10;
l2.val = sum % 10;
prev.next = l2;
prev = l2;
l2 = l2.next;
}
}
if (carry > 0) {
prev.next = new ListNode(carry);
}
return ret;
}
```