Kth Smallest Element in a BST

class Solution {
    public List<Integer> inorder(TreeNode node, List<Integer> arr) {
        if (node == null) return arr;
        inorder(node.left, arr);
        arr.add(node.val);
        inorder(node.right, arr);
        return arr;
    }
    public int kthSmallest(TreeNode root, int k) {
        List<Integer> arr = inorder(root, new ArrayList<Integer>());
        return arr.get(k-1);
    }
}