Solution Review: Finding Kth Maximum Value in Binary Search Tree

using System;
namespace chapter_6
{
    class Solution
    {
        //Helper recursive function to traverse tree using inorder traversal
   
        static void inOrderTraversal(Node rootNode, ArrayList result)
        {
            if (rootNode != null)
            {
                inOrderTraversal(rootNode.leftChild, result);
                result.insert(rootNode.value);
                inOrderTraversal(rootNode.rightChild, result);
            }
        }
        static int findKthMax(Node rootNode, int k, int nodes)
        {
            if (k <= 0)
            {
                return -1;
            }
            //Perform In-Order Traversal to get sorted array. (ascending order)
            //Return value at index [length - k]
            ArrayList result = new ArrayList(nodes);
            inOrderTraversal(rootNode, result);
            return result.getAt(nodes - k);
        }
        static void Main(string[] args)
        {
            BinarySearchTree bsT = new BinarySearchTree(6);
           bsT.insertBST(4);
           bsT.insertBST(9);
           bsT.insertBST(5);
           bsT.insertBST(2);
           bsT.insertBST(8);
          Console.WriteLine(findKthMax(bsT.getRoot(), 3, 6));
          return;
        }
    }
}