2. Add Two Numbers

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

两个链表相加：可以分为指针操作和值操作，指针有dummy, head, l1, l2，分别代表结果，l1，l2链表，值操作包括余数和进位问题，一个是%10，一个是/10，边界条件包括三个，进位为1的时候还要加一次carry。

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode dummy = new ListNode(0);
    ListNode head = dummy;
    int carry = 0;
    while (l1 != null || l2 != null || carry == 1) {
        int v1 = (l1 != null) ? l1.val : 0;
        int v2 = (l2 != null) ? l2.val : 0;
        int value = v1 + v2 + carry;
        head.next = new ListNode(value % 10);
        carry = value / 10;

        if (l1 != null) l1 = l1.next;
        if (l2 != null) l2 = l2.next;
        head = head.next;
    }
    return dummy.next;
}