Palindrome Linked List

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

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

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) {
            return true;
        }
        ListNode p1 = head;
        ListNode p2 = head;
        while (p2.next != null && p2.next.next != null) {
            p1 = p1.next;
            p2 = p2.next.next;
        }
        ListNode head2 = p1.next;
        p1.next = null;
       
        // reverse head2;
        ListNode pre = null;
        ListNode p = head2;
        while (p != null) {
            ListNode nextP = p.next;
            p.next = pre;
           
            pre = p;
            p = nextP;
        }
        ListNode newHead2 = pre;
       
        p1 = head;
        p2 = newHead2;
        while (p1 != null && p2 != null) {
            if (p1.val != p2.val) {
                return false;
            }
            p1 = p1.next;
            p2 = p2.next;
        }
        return true;
    }
}