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. The function should return the predicted temperature.
  
  def predict_temperature(temperature_list):
      assert predict_temperature([21, 22, 20])==21.5
      assert predict_temperature([25, 23, 24])==23.5
      assert predict_temperature([20, 20, 20])==20.0
      assert predict_temperature([10, 15, 20])==15.0
output: def predict_temperature(temperature_list):
      alpha = 0.5
      smoothed_temp = alpha * temperature_list[-1] + (1 - alpha) * (alpha * temperature_list[-2] + (1 - alpha) * temperature_list[-3])
      return smoothed_temp