请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false
示例 2:

输入: 1->2->2->1
输出: true
进阶：
你能否用 O(n) 时间复杂度和 O(1) 空间复杂度解决此题？

思路：逆置前一半，然后从中心出发开始比较即可。

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode(int x) { val = x; }* }*/
class Solution {public boolean isPalindrome(ListNode head) {if(head == null || head.next == null) {return true;}ListNode slow = head, fast = head;ListNode pre = head, prepre = null;while(fast != null && fast.next != null) {pre = slow;slow = slow.next;fast = fast.next.next;pre.next = prepre;prepre = pre;}if(fast != null) {slow = slow.next;}while(pre != null && slow != null) {if(pre.val != slow.val) {return false;}pre = pre.next;slow = slow.next;}return true;}
}