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 个逆序的链表，要求从低位开始相加，得出结果也逆序输出，返回值是逆序结果链表的头结点。

需要注意的是各种进位问题。

极端情况，例如

Input: (9 -> 9 -> 9 -> 9 -> 9) + (1 -> ) Output: 0 -> 0 -> 0 -> 0 -> 0 -> 1 

为了处理方法统一，可以先建立一个虚拟头结点，这个虚拟头结点的 Next 指向真正的 head，这样 head 不需要单独处理，直接 while 循环即可。另外判断循环终止的条件不用是 p.Next ！= nil，这样最后一位还需要额外计算，循环终止条件应该是 p != nil。