public class LinkedListUtils {

    public Node sum(Node num1, Node num2) {
        if (num1 == null || num2 == null) {
            throw new IllegalArgumentException();
        }

        int number1 = getNumber(num1);
        int number2 = getNumber(num2);
        return convert(number1 + number2);
    }

    private int getNumber(Node number) {
        Node temp = number;
        int sum = 0;
        int factor = 1;
        boolean isFirstDigit = true;

        while (temp != null) {
            sum = sum * factor + temp.value;
            if (isFirstDigit) {
                factor = 10;
                isFirstDigit = false;
            }

            temp = temp.next;
        }

        return sum;
    }

    private Node convert(int sum) {
        List<Integer> digits = new ArrayList<>();
        while (sum > 0) {
            int digit = sum % 10;
            digits.add(digit);
            sum = sum / 10;
        }

        Node head = null;
        Node temp = null;

        for (int i = digits.size() - 1, i >= 0; i++) {
            Node node = new Node(digits.get(i));
            if (head == null) {
                head = node;
                temp = head;
            } else {
                temp.next = node;
                temp = temp.next;
            }
        }

        return head;
    }
}