You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

/*
 * 2. Add Two Numbers
 * https://leetcode.com/problems/add-two-numbers/
 * https://realneo.me/2-add-two-numbers/
 */

public class AddTwoNumbers {
    public static void main(String[] args) {
        ListNode l1 = new ListNode(2);
        l1.next = new ListNode(4);
        l1.next.next = new ListNode(3);

        ListNode l2 = new ListNode(5);
        l2.next = new ListNode(6);
        l2.next.next = new ListNode(4);

        AddTwoNumbers solution = new AddTwoNumbers();

        ListNode head = solution.addTwoNumbers(l1, l2);
        solution.print(head);
    }

    private void print(ListNode node) {
        for (; node != null; node = node.next) {
            System.out.print(node.val);
            System.out.print("->");
        }
        System.out.println("null");
    }

    private ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode node = new ListNode(0);
        ListNode head = node;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int val = 0;
            if (l1 != null) {
                val += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                val += l2.val;
                l2 = l2.next;
            }
            val += carry;
            node.next = new ListNode(val % 10);
            carry = val / 10;
            node = node.next;
        }
        if (carry > 0)
            node.next = new ListNode(carry);

        return head.next;
    }
}