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; }
 * }
 */
class Solution {
    public boolean isPalindrome(ListNode head) {
        if (head == null) return true;
        ListNode slow = head, fast = head;

        while (fast != null && fast.next != null){
            slow = slow.next;
            fast = fast.next.next;
        }

        if (fast != null){
            slow = slow.next;
        }

        slow = reverse(slow);
        fast = head;

        while (slow != null){
            if (slow.val != fast.val){
                return false;
            }

            slow = slow.next;
            fast = fast.next;
        }

        return true;
    }

    private ListNode reverse(ListNode head){
        ListNode prev = null, cur = head;

        while (cur != null){
            ListNode next = cur.next;
            cur.next = prev;
            prev = cur;
            cur = next;
        }

        return prev;
    }
}