public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        ListNode p = l1;
        ListNode q = l2;
        ListNode dummyHead = new ListNode(0);


        ListNode curr = dummyHead;
        int carry = 0;
        while (p != null || q!= null) {
            int x = p != null ? p.val : 0;
            int y = q != null ? q.val : 0;

            int sum = carry + x + y;

            carry = sum / 10;

            curr.next = new ListNode(sum % 10);

            curr = curr.next;


            if (p != null) p = p.next;
            if (q != null) q = q.next;
        }

        //take care of last sum carry to final result 
        if (carry > 0) {
            curr.next = new ListNode(carry);
        }

        return dummyHead.next;

    }