题目：
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.
列子：
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4) Output: 7 -> 0 -> 8 Explanation: 342 + 465 = 807.
题目的大概意思是：一个整数如342，在链表中存放形式是倒着的（即：2->4->3），再如465，
在链表中的存放形式是：5->6->4。将这两个数相加得807，存放到链表中表现为7->0->8。
