public void preOrder() { preOrder(this.root); }

    private void preOrder(BSTNode node) {
        if (node != null) {
            System.out.print(node.getKey() + " ");
            preOrder(node.getLeft());
            preOrder(node.getRight());
        }
    }

    public void inOrder() { inOrder(this.root); }

    private void inOrder(BSTNode node) {
        if (node != null) {
            inOrder(node.getLeft());
            System.out.print(node.getKey() + " ");
            inOrder(node.getRight());
        }
    }

    public void postOrder() { postOrder(this.root); }

    private void postOrder(BSTNode node) {
        if (node != null) {
            postOrder(node.getLeft());
            postOrder(node.getRight());
            System.out.print(node.getKey() + " ");
        }
    }