input: Write a python function to parse a JSON string and separate city names by their continents.
  assert separate_cities('{"locations":[{"name":"New York","continent":"North America"},{"name":"Los Angeles","continent":"North America"},{"name":"Paris","continent":"Europe"},{"name":"Berlin","continent":"Europe"}]}') == (["New York", "Los Angeles"], ["Paris", "Berlin"])
  assert separate_cities('{"locations":[{"name":"San Francisco","continent":"North America"},{"name":"London","continent":"Europe"}]}') == (["San Francisco"], ["London"])
output: def separate_cities(json_string):
      import json
      data = json.loads(json_string)
      north_america = []
      europe = []
      for location in data['locations']:
          if location['continent'] == 'North America':
              north_america.append(location['name'])
          elif location['continent'] == 'Europe':
              europe.append(location['name'])
      return north_america, europe