input: Write a python function to create a Flask application that returns "Hello World!" at the root route.
  assert hello_world_app().test_client().get('/').data == b"Hello World!"
  assert isinstance(hello_world_app(), Flask)
output: def hello_world_app():
      from flask import Flask
      
      app = Flask(__name__)
      
      @app.route('/')
      def hello():
          return "Hello World!"
      
      return app