//My Code School solution
 public bool IsBST(BSTNode <int> root) =>
 IsBSTUtil(root, int.MinValue, int.MaxValue);
        public void AddNodeRecursive(int x)
        {
            var refNode = Root;

            Root = AddNode(x, refNode);
        }
        public bool IsBinarySearchTree(BSTNode <int> root)
        {
            int _value = int.MinValue;

            return(IsBST(root, ref _value));
        }