LeetCode 234: Palindrome Linked List

/**
 * 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;
        }
        
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        
        fast = reverse(slow.next);
        slow.next = null;
        slow = head;
        
        while (fast != null) {
            if (fast.val != slow.val) {
                return false;
            }
            fast = fast.next;
            slow = slow.next;
        }
        
        return true;
    }
    
    private ListNode reverse(ListNode head) {
        ListNode next = head.next;
        ListNode prev = null;
        
        while (head != null) {
            next = head.next;
            head.next = prev;
            prev = head;
            head = next;
        }
        return prev;
    }
}