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 ListNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @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;
        // 使用快慢指针找到中点位置
        ListNode slow = head, fast = head.next;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        // 翻转后半部分链表
        fast = reverse(slow.next);
        slow.next = null;
        slow = head;
        while (fast != null && slow != null) {
            if (slow.val != fast.val) return false;
            slow = slow.next;
            fast = fast.next;
        }
        return true;
    }
    private ListNode reverse(ListNode head) {
        ListNode pre = null, cur = head, next;
        while (cur != null) {
            next = cur.next;
            cur.next = pre;
            pre = cur;
            cur = next;
        }
        return pre;
    }
}