input: Write a python function to generate a response based on predefined conversations.
  assert answer('Hello') in ['Hi there']
  assert answer('How are you?') in ['I am doing great!']
  assert answer('Bye') in ['Goodbye!']
  assert answer('What is your name?') == ''
output: def answer(input_string):
      responses = {
          'Hello': 'Hi there',
          'How are you?': 'I am doing great!',
          'Bye': 'Goodbye!'
      }
      return responses.get(input_string, '')