private static <T> void left_boundary_b_tree(BinaryTree<T> n, boolean is_boundary) {
        if(n != null) {
            if(is_boundary || (n.getLeft() == null && n.getRight() == null)) {
                System.out.print(n.getData() + " ");
            }
            left_boundary_b_tree(n.getLeft(), is_boundary);
            left_boundary_b_tree(n.getRight(), is_boundary && n.getLeft() == null);
        }
    }