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.

addTwoNumbers.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 * example:
 * Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
 * Output: 7 -> 0 -> 8
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return addTwoNumbersHelper(l1, l2, 0);
    }
    
    public ListNode addTwoNumbersHelper(ListNode l1, ListNode l2, int carry) {
        ListNode node = null;
        int sum = 0;
        if(l1 == null && l2 == null) {
            if(carry > 0) {
                node = new ListNode(carry);
                return node;
            } else {
                return null;
            }
        }
        int l1Val = l1 != null? l1.val:0;
        int l2Val = l2 != null? l2.val:0;
        
        sum = l1Val + l2Val + carry;
        if(sum >= 10) {
            carry = 1;
            sum -= 10;
        } else {
            carry = 0;
        }
        node = new ListNode(sum);
        node.next = addTwoNumbersHelper(l1 != null ? l1.next : null,
                                        l2 != null ? l2.next : null,
                                        carry);
                                        
        return node;
    }
}