# Palindrome Linked List

Given a singly linked list, determine if it is a palindrome.

## Example

**Example 1:**

```
Input:
 1->2

Output:
 false
```

**Example 2:**

```
Input:
 1->2->2->1

Output:
 true
```

**Follow up:**\
Could you do it in O(n) time and O(1) space?

## Note

找中点+reverse+两个头指针比较

## Code

```java
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        if(head == null || head.next == null) return true;
        // if(head.next.next == null && head.val != head.next.val)
        //     return false;
        ListNode middle = findMiddle(head);
        ListNode p = head;
        ListNode q = reverse(middle);
        while (q != null) {
            if (q.val != p.val)
                return false;
            q = q.next;
            p = p.next;
        }
        return true;
    }

    public ListNode findMiddle(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;        
    }

    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = prev;
            prev = head;
            head = temp;
        }
        return prev;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://luj.gitbook.io/code/string/6-palindrome/palindrome-linked-list.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.