题解 | #判断一个链表是否为回文结构#

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 the head
     * @return bool布尔型
     */
    public boolean isPail (ListNode head) {
        if(head == null || head.next == null){
            return true;
        }

        ListNode fast = head, slow = head;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }

        if(fast != null){
            slow = slow.next;
        }

        fast = head;
        ListNode reHead = reverse(slow);
        while(reHead != null){
            if(reHead.val != fast.val){
                return false;
            }

            reHead = reHead.next;
            fast = fast.next;
        }

        return true;
    }

    private static ListNode reverse(ListNode head){
        if(head == null){
            return null;
        }

        ListNode pre = null, cur = head;
        while(cur != null){
            ListNode tmp = cur.next;
            cur.next = pre;
            pre = cur;
            cur = tmp;
        }

        return pre;
    }
}