Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth 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?

public class Solution {    public int kthSmallest(TreeNode root, int k) {        int count = countNodes(root.left);        if (k <= count) {            return kthSmallest(root.left, k);        } else if (k > count + 1) {            return kthSmallest(root.right, k-1-count); // 1 is counted as current node        }        return root.val;    }    public int countNodes(TreeNode n) {        if (n == null) return 0;        return 1 + countNodes(n.left) + countNodes(n.right);    }}