Given a binary search tree, write a function
kthSmallest to find the kth smallest element in it.
You may assume k is always valid, 1 ≤ k ≤ BST's total elements.
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?
Special thanks to @ts for adding this problem and creating all test cases.