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/
