回文链表

题目描述：
请判断一个链表是否为回文链表。

示例：
输入: 1->2->2->1
输出: true

Java代码：

/**
 * 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 fast = head, slow = 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;
    }
}