Insert into a Cyclic Sorted List

思路：
三种情况。
1. x在最大和最小值之间，之间插入。
2. x大于最大数，插入最大和最小之间。
3. x小于最小数，同2。

实现：
在node  == null的时候，记得让新的node.next指向自己。因为是循环链表。

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