input: Write a python function to simulate a state-based conversation system.
  assert isinstance(conversation_system(['I need some advice', 'Career advice']), list)
  assert 'Sure, what kind of advice do you need?' in conversation_system(['I need some advice'])
  assert 'Goodbye!' in conversation_system(['I need some advice', 'Career advice'])
  assert 'I don\'t understand what you mean. Please try again.' in conversation_system(['I need some advice', 'Unknown'])
output: def conversation_system(inputs):
      state = 'start'
      responses = []
      
      for input in inputs:
          if state == 'start':
              if 'advice' in input.lower():
                  responses.append('Sure, what kind of advice do you need?')
                  state = 'awaiting_advice_type'
              else:
                  responses.append('I don\'t understand what you mean. Please try again.')
          elif state == 'awaiting_advice_type':
              if 'career' in input.lower():
                  responses.append('I think you should focus on networking and improving your skills.')
                  state = 'end'
              elif 'relationship' in input.lower():
                  responses.append('Communication and understanding are key.')
                  state = 'end'
              else:
                  responses.append('I don\'t understand what you mean. Please try again.')
          elif state == 'end':
              responses.append('Goodbye!')
              break
      
      if state == 'awaiting_advice_type':
          responses.append('I don\'t understand what you mean. Please try again.')
          
      return responses