def warning(message, **kwargs):
    kwargs["fg"] = kwargs.get("fg", "red")
    click.secho(u"warning: {}".format(message), **kwargs)
def dfs_print(root):
        if root is None:
            return
        dfs_print(root.left)
        print(str(root.key) + ' ', end='')
        dfs_print(root.right)