message: Update design-underground-system.cpp
diff: -// Time: O(n)
  +// Time: ctor: O(1)
  +// checkin: O(1)
  +// checkout: O(1)
  +// getaverage: O(1)
  // Space: O(n)
  class UndergroundSystem {
  @@ -18,8 +21,8 @@ public:
  }
  double getAverageTime(string startStation, string endStation) {
  - const auto& [numer, denom] = statistics_[startStation][endStation];
  - return double(numer) / denom;
  + const auto& [total_time, cnt] = statistics_[startStation][endStation];
  + return double(total_time) / cnt;
  }
  private:
  