不使用额外空间，手动使用尾插法创建单链表并实现判断回文串功能

import java.util.Scanner;
//定义链表结构
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 static void main(String[] args) {
    	//控制台输入使用尾插法创建链表结构
        Scanner sc = new Scanner(System.in);
        ListNode newNode = new ListNode();
        ListNode head = newNode;
        head.next = null;
        ListNode tail = head;
        while(sc.hasNext()){
            int val = sc.nextInt();
            if(val == -1){
                break;
            }
            ListNode nextNode = new ListNode(val);
            tail.next = nextNode;
            tail = nextNode;
        }
        System.out.println(isPalindrome(head.next));
    }
    //不使用额外空间实现判断回文链表的功能，适用与面试时写
    public static boolean isPalindrome(ListNode head){
		//使用快慢指针的思想
        ListNode slow = head;
        ListNode fast = head;
        //定义额外指针实现反转链表
        ListNode temp = null;
        while(fast.next != null && fast.next.next != null){
        	//慢指针走一步，快指针走两步，当快指针走到最后的时候慢指针恰好在链表中点
            slow = slow.next;
            fast = fast.next.next;
        }
        //链表中点右半部分反转
        fast = slow.next;
        slow.next = null;
        while(fast != null){
            temp = fast.next;
            fast.next = slow;
            slow = fast;
            fast = temp;
        }
        //为了最后还原链表
        temp = slow;
        //上边反转链表后fast为null，slow指向原链表最后的节点
        fast = head;
        boolean res = true;
        //进行回文链表判断
        while(slow != null && fast != null){
            if(slow.val != fast.val){
                res = false;
                break;
            }
            slow = slow.next;
            fast = fast.next;
        }
        //还原为原链表
        slow = temp.next;
        temp.next = null;
        while(slow != null){
            fast = slow.next;
            slow.next = temp;
            temp = slow;
            slow = fast;
        }
        //验证原链表是否还原
//        while(head != null){
//            System.out.print(head.val + " ");
//            head = head.next;
//        }
        return res;
    }
}