61.回文链表

给你一个单链表的头节点 head ，请你判断该链表是否为回文链表。如果是，返回 true ；否则，返回 false 。

/**
 * 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;
        }
        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;
    }
}