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])==21.0
  assert predict_temperature([18, 20, 19])==19.0
  assert predict_temperature([25, 24, 23])==23.5
  assert predict_temperature([10, 12, 11])==11.0
  assert predict_temperature([])==None
output: def predict_temperature(past_temps):
      if not past_temps:
          return None
      alpha = 0.5
      smoothed_temp = past_temps[0]
      for temp in past_temps[1:]:
          smoothed_temp = alpha * temp + (1 - alpha) * smoothed_temp
      return smoothed_temp