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)
