题解 | #判断一个链表是否为回文结构#

/*
 * 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) {
        // write code here
        if(head == null || head.next == null ){
            return true;
        }
        //快慢指针
        ListNode slow = head;
        ListNode fast = head;
        ListNode pre = null;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            ListNode next = slow.next;
            slow.next = pre;
            pre = slow;
            slow = next;
        }
        //偶数 fast = null 
        //奇数时 fast != null slow要后移一位
        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;
    }
}