234. 回文链表

    public boolean isPalindrome(ListNode head) {
        if (head == null || head.next == null) {
            return true;
        }
        ListNode slow = head, fast = head.next, pre = null, ppre = null;
        while (fast != null && fast.next != null) {
            pre = slow;
            // 后移
            slow = slow.next;
            fast = fast.next.next;
            // 反向指针
            pre.next = ppre;
            ppre = pre;
        }
        ListNode p2 = slow.next;
        slow.next = pre;
        ListNode p1 = fast == null ? slow.next : slow;
        while (p1 != null) {
            if (p1.val != p2.val) {
                return false;
            }
            p1 = p1.next;
            p2 = p2.next;
        }
        return true;
    }