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

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

题目意思就是两个链表代表了两个整数做加法，第一个是最低位个位，第二个是十位。。。依次，和正常的数字刚好相反，因为正常数字第一个数是最高位。再注意一下进制就可以了 3 4 5 3 1 4

res = new node(6) head = res tail = res

res = new node(5) tail.next = res tail = res

res = new node(9) tail.next = res tail = res