使用快慢指针将链表切成两段，然后对后面的一段进行反转，再逐个比较两个子链。
Given a singly linked list, determine if it is a palindrome.
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?
link:{sourcedir}/_0234_PalindromeLinkedList.java[role=include]