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. The function should return the predicted temperature. If the input list is empty, the function should return None.
  assert predict_temperature([21, 22, 20])==21.0
  assert predict_temperature([19, 23, 21])==20.5
  assert predict_temperature([25, 24, 23])==23.25
  assert predict_temperature([])==None
  assert predict_temperature([30, 32, 29])==27.625
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