input: Write a function to add and retrieve users using Flask.
  assert add_user({"userId": 1, "name": "John"}) == {'message': 'User added successfully'}
  assert get_user(1) == {'user': {"userId": 1, "name": "John"}}
  assert get_user(2) == {'message': 'No user found'}
output: from flask import Flask, request, jsonify
  
  app = Flask(__name__)
  users = {}
  
  @app.route('/add_user', methods=['POST'])
  def add_user():
      user = request.json
      user_id = user['userId']
      users[user_id] = user
      return jsonify({'message': 'User added successfully'})
  
  @app.route('/get_user/<int:user_id>', methods=['GET'])
  def get_user(user_id):
      user = users.get(user_id)
      if user:
          return jsonify({'user': user})
      else:
          return jsonify({'message': 'No user found'})
  
  if __name__ == '__main__':
      app.run(debug=True)