Given a singly linked list, determine if it is a palindrome. 给出一个单链表，确定它是否是回文。

Example 1:

Input: 1->2

Output: false

Example 2:

Input: 1->2->2->1 Output: true

Follow up:

Could you do it in O(n) time and O(1) space?

• 思路 把链表转化成数组，然后分别从数组的头部和尾部开始比较

• 用时 30分钟