public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(0);
        ListNode p = dummy;

        int carry = 0;
        while(l1 != null || l2 != null || carry != 0){
            int sum = carry;
            if(l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                sum += l2.val;
                l2 = l2.next;
            }
            carry = sum / 10;
            ListNode node = new ListNode(sum % 10);
            p.next = node;
            p = p.next;
        }
        return dummy.next;
    }