用到快慢指针！

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public boolean isPalindrome(ListNode head) {if(head == null || head.next == null)return true;//空链表和一个节点链表也是回文链表ListNode slow = head;ListNode fast = head.next;while (fast != null && fast.next != null){slow = slow.next;fast = fast.next.next;}ListNode newHead = slow.next;slow.next = null;ListNode newFastHead = reverseList(newHead);ListNode right = newFastHead;ListNode left = head;while (right != null){if(right.val != left.val){return false;}left = left.next;right = right.next;}return true;}public ListNode reverseList(ListNode head){ListNode prev = null;ListNode curr = head;while (curr != null){ListNode next = curr.next;curr.next = prev;prev = curr;curr = next;}return prev;}}