input: Write a function to predict the next day's forecast temperature using the exponential smoothing algorithm given the past three day's forecast temperatures. If the provided list is empty, the function should return None.
  assert predict_temperature([21, 22, 20])==21.0
  assert predict_temperature([18, 19, 17])==17.5
  assert predict_temperature([25, 24, 26])==25.0
  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_value = past_temps[0]
      for temp in past_temps:
          smoothed_value = alpha * temp + (1 - alpha) * smoothed_value
      return smoothed_value