Given a binary search tree, write a functionkthSmallestto find thekth smallest element in it.

Note:

You may assume k is always valid, 1 ≤ k ≤ BST's total elements.

Follow up:

What if the BST is modified (insert/delete operations) often and you need to find the kth smallest frequently? How would you optimize the kthSmallest routine?

第一种方法 中序遍历到k 返回。 第二种递归。 这个新颖。但是两次递归， 有点坑爹/