You are given two linked lists representing two non-negative numbers. 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

要求不能改動input， 也就是說要建立一個新的list， 第一種方法借用兩個棧， 構(gòu)建一個新的list， 這樣空間復(fù)雜度高。

第二種方法就是借鑒+1 的記錄最后一個不是9的位置，?