CTCI 2.5

You have two numbers represented by a linked list, where each node contains a single digit. The digits are stores in reverse order, such 
that the 1's digit is at the head of the list. Write a function that adds the two numbers and returns the sum as a linked list.
FOLLOW UP
Suppose the digits are stored in forward order. Repeat the above problem.

CTCI 2.5 reverseOrder.java

public static Node addReverseOrderList(Node firstNumber, Node secondNumber){
    return addReverseOrderList(firstNumber, secondNumber, 0);
}
public static Node addReverseOrderList(Node firstNumber, Node secondNumber, int carry){
    if(firstNumber == null && secondNumber == null){
        if(carry == 1) return new Node(1);
        else return null;
    }
    int sum = 0;
    if(firstNumber != null) sum += firstNumber.value;
    if(secondNumber != null) sum += secondNumber.value;
    sum += carry;
    Node nextNode = addReverseOrderList(
        firstNumber != null ? firstNumber.next : null,
        secondNumber != null ? secondNumber.next : null,
        sum >= 10 ? 1 : 0);
    Node thisNode = new Node(sum % 10);
    thisNode.next = nextNode;
    return thisNode;
}