Add Two Numbers

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Recursive-Code

//using global variable in the recursion is not graceful
    //I prefer to use a help_function
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        return addTwoNumbers_helper(l1, l2, 0);
    }

    public ListNode addTwoNumbers_helper(ListNode l1, ListNode l2, int carry) {
        if (l1 == null && l2 == null) {
            //here we need to handle the carry
            if (carry == 0)
                return null;
            else {
                ListNode a = new ListNode(carry);
                return a;
            }

        }
        int first = 0, second = 0;
        if (l1 != null)
            first = l1.val;
        if (l2 != null)
            second = l2.val;
        ListNode result = new ListNode((first + second + carry) % 10);
        carry = (first + second + carry) / 10;
        result.next = addTwoNumbers_helper(l1 == null ? null : l1.next,
                l2 == null ? null : l2.next, carry
        );
        return result;

    }

Non-recursive

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1 == null && l2 == null)
            return null;
        int first = l1.val;
        int second = l2.val;

        ListNode l3 = new ListNode((first + second) % 10);
        ListNode output = l3;
        int carry = (first + second) / 10;
        l1 = l1.next;
        l2 = l2.next;
        //at this point, the list node may be null,
        //pay attention to the null condition
        while (l1 != null || l2 != null) {
            if (l1 == null)
                first = 0;
            else {
                first = l1.val;
                l1 = l1.next;
            }
            if (l2 == null) {
                second = 0;
            } else {
                second = l2.val;
                l2 = l2.next;
            }
            l3.next = new ListNode((first + second + carry) % 10);
            carry = (first + second + carry) / 10;
            l3 = l3.next;
        }
        //don't forget the carry
        if (carry != 0)
            l3.next = new ListNode(carry);
        return output;
    }