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.
思路：
1、实现链表的加法
2、设定一个dummy节点、用来保存两个数字之和的链表
3、第一个循环、两个链表都不为空、得出进位和余数、进位要在最外面初始化为0
4、第二个循环、如果第一个链表不为空接着加
5、第三个循环、如果第二个链表不为空接着加
6、最后要判断进位如果不等于0，把进位加上
