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Given a binary search tree, write a function `kthSmallest`

to find the **k**th 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?

**Credits:**

Special thanks to @ts for adding this problem and creating all test cases.

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