25K个一组翻转链表

 public ListNode reverseKGroup(ListNode head, int k) {
        // 1 边界条件
        ListNode node = head;
        int count = 0;
        while (count < k) {
            if (node == null) {
                return head;
            }
            count++;
            node = node.next;
        }
        // 2 下潜
        ListNode pre = reverseKGroup(node, k);
        // 3 解决子问题
        while (0 < count--) {
            ListNode next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }