class App {
  /**
   * Program entry point.
   *
   * @param args command line args
   */
  public static void main(String[] args) {<FILL_FUNCTION_BODY>}
}

    var root = new NodeImpl("1",
        new NodeImpl("11",
            new NodeImpl("111", NullNode.getInstance(), NullNode.getInstance()),
            NullNode.getInstance()
        ),
        new NodeImpl("12",
            NullNode.getInstance(),
            new NodeImpl("122", NullNode.getInstance(), NullNode.getInstance())
        )
    );

    root.walk();
  
<no_super_class>