message: Update design-underground-system.py
diff: -# Time: O(n)
  +# Time: ctor: O(1)
  +# checkin: O(1)
  +# checkout: O(1)
  +# getaverage: O(1)
  # Space: O(n)
  import collections
  @@ -37,5 +40,5 @@ class UndergroundSystem(object):
  :type endStation: str
  :rtype: float
  """
  - numer, denom = self.__statistics[startStation, endStation]
  - return float(numer) / denom
  + total_time, cnt = self.__statistics[startStation, endStation]
  + return float(total_time) / cnt
  