234. 回文链表

请判断一个链表是否为回文链表。

示例 1:

输入: 1->2
输出: false

示例 2:

输入: 1->2->2->1
输出: true

import java.util.ArrayList;

public class PalindromeLinkedList {
    static class ListNode {
        int val;
        ListNode next;
        ListNode(int x) {
            val = x;
        }
    }
    
    //方法一：遍历链表，将数据存储在集合中，然后将集合的头尾进行遍历
    public boolean isPalindrome(ListNode head) {
        ListNode current = head;
        ArrayList<Integer> arr = new ArrayList<>();
        while(current != null) {
            arr.add(current.val);
            current = current.next;
        }
        int len = arr.size();
        int first = 0;
        int last = len-1;
        while(first<last) {
            if(!arr.get(first).equals(arr.get(last))) {
                return false;
            }
            first++;
            last--;
        }
        return true;
    }
    
    //方法二：快慢指针查找链表的中点,将链表切断为两部分，对后一半的链表进行反转，再与前一半的链表进行比较
    public boolean isPalindrome2(ListNode head) {
        if(head == null || head.next == null) {
            return true;
        }
        ListNode slow = head;
        ListNode fast = head;
        //利用快慢指针查找中间节点，一个移动1个节点，另一个移动两个节点，当快指针移动到末尾时，慢指针移动到中间
        while(fast != null && fast.next != null) {
            slow = slow.next;  //当fast为null时，节点数为偶数，slow为中间偏右
            fast = fast.next.next;  //当fast不为null时，节点数为奇数，slow为正中间
        }
        cutLinkedList(head, slow);
        ListNode backHalf = reverse(slow);
        while(head != null && backHalf != null) {
            if(head.val != backHalf.val) {
                return false;
            }
            head = head.next;
            backHalf = backHalf.next;
        }
        return true;
    }
    public static void cutLinkedList(ListNode head, ListNode cutNode) {
        while(head.next != cutNode) {
            head = head.next;
        }
        head.next = null;
    }
    public static ListNode reverse(ListNode head) {
        ListNode newHead = null;
        while(head != null) {
            ListNode next = head.next; 
            head.next = newHead;
            newHead = head;
            head = next;
        }
        return newHead;
    }
}