1class Solution {2    public boolean isPalindrome(ListNode head) {3        if (head == null || head.next == null) return true;4        ListNode slow = head, fast = head;5        while (fast.next != null && fast.next.next != null) {6            slow = slow.next;7            fast = fast.next.next;8        }9        ListNode second = reverse(slow.next);10        ListNode a = head, b = second;11        boolean answer = true;12        while (b != null) {13            if (a.val != b.val) answer = false;14            a = a.next; b = b.next;15        }16        slow.next = reverse(second);17        return answer;18    }19    private ListNode reverse(ListNode current) {20        ListNode prev = null;21        while (current != null) {22            ListNode next = current.next;23            current.next = prev;24            prev = current; current = next;25        }26        return prev;27    }28}