2. Add Two Numbers 「两数相加」

给定两个非空链表，分别代表着两个非负整数。整数的每一位数字以倒序保存在链表的每一个节点中。将这两个整数相加并将结果以相同形式的链表返回。

你可以假设这两个整数最高位不为 0，除非是遇到 0 本身这个数。

举例：
输入： (2 -> 4 -> 3) + (5 -> 6 -> 4)
输出： 7 -> 0 -> 8
解释： 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;
    }
}