input: Write a function to predict the next day's forecasted temperature using the exponential smoothing algorithm given the past three day's forecasted temperatures. If the input list is empty, return None.
  assert predict_temperature([21, 22, 20]) == 20.5
  assert predict_temperature([19, 20, 21]) == 20.5
  assert predict_temperature([20, 20, 20]) == 20.0
  assert predict_temperature([]) == None
output: def predict_temperature(past_temps):
      if not past_temps:
          return None
      alpha = 0.5
      smoothed_temp = past_temps[-1]
      for temp in past_temps[-2::-1]:
          smoothed_temp = alpha * temp + (1 - alpha) * smoothed_temp
      return smoothed_temp