Convert Sorted List to Balanced BST

思路：
用merge sort的思想来做很简单，时间复杂度是O(nlogn)。因为没一次递归都要遍历链表找中点。

另一种是inorder traversal, 实在是太巧妙了，估计面试的时候想不出来。

实现：
关于找中点slow和fast初始化问题, fast =  head or fast  =  head.next对于最后结果并没有什么影响。

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