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;
        }
        ListNode cur = head;
        ListNode pre = head;
        while (pre != null && pre.next != null) {
            cur = cur.next;
            pre = pre.next.next;
        }
        // 获取链表的中间位置,并且反转
        cur =  reversalList(cur);
        // 当前结点
        ListNode node = head;
        while (cur != null) {
            if (node.val != cur.val) {
                return false;
            }
            node = node.next;
            cur = cur.next;
        }
        return true;
    }

    public ListNode reversalList(ListNode head) {
        // 定一个栈, 将head1进行反转
        Stack<ListNode> stack = new Stack<>();
        while (head != null) {
            stack.push(head);
            head = head.next;
        }
        // 出栈
        head = stack.pop();
        ListNode cur = head;
        while (!stack.isEmpty()) {
            ListNode node = stack.pop();
            node.next = null;
            cur.next = node;
            cur = cur.next;
        }
        return head;
    }
}