class Solution {
    // 2. Add Two Numbers
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        ListNode dummy = new ListNode(0);
        ListNode l3 = dummy;
        int sum = 0;

        while (l1 != null || l2 != null || sum != 0) {
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }

            l3.next = new ListNode(sum % 10);
            l3 = l3.next;
            sum /= 10;
        }
        return dummy.next;
    }
}