445. Add Two Numbers II(链表求和)

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 8 -> 0 -> 7

方法一：这个题涉及到加法运算。记得设置进位，从链表尾开始进行加法运算，所以我得想办法从尾部向前运算，链表是单向，所以想到栈结构，后进先出。最后返回的是链表，
所以得设置两个指针，一个标记链表头，另一个参与运算。

时间复杂度：o（n）                 空间复杂度：o(n)