Palindrome Linked List

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

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?

class Solution {
    public ListNode reverseList(ListNode A) {
        ListNode prev = null, current = A;
        while (current.next != null) {
            ListNode temp = current.next;
            current.next = prev;
            prev = current;
            current = temp;
        }
        current.next = prev;
        return current;
    }

    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null)
            return true;
        ListNode slow = head;
        ListNode fast = head;
        while (fast != null && fast.next != null) {
            if (fast.next.next == null) {
                break;
            }
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode right = slow.next;
        slow.next = null;
        ListNode left = reverseList(head);
        if (fast != slow && fast.next == null)
            left = left.next;
        while (left != null && right != null) {
            if (left.val != right.val)
                return false;
            left = left.next;
            right = right.next;
        }
        return true;
    }
}