8.3 LeetCode230 Return the kth smallest element of 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?

Method: According to the property of BST, the inOrder order is just the ascending order. So insert the BST as inOrder into an ArrayList then return list(k-1).

public class Solution { // the inOrder sequence is the ascending order.
    ArrayList<Integer> list = new ArrayList<Integer>();
    public int kthSmallest(TreeNode root, int k) {
        insertArray(root);
        return list.get(k-1);
    }
    private void insertArray(TreeNode root) {
        if(root.left != null) insertArray(root.left);
        list.add(root.val);
        if(root.right != null) insertArray(root.right);
    }
}