input: from typing import List
  
  def predict_temperature(temperature_list: List[float]) -> float:
      """ Using exponential smoothing algorithm, predict the next day's forecasted temperature given the past three day's forecasted temperatures.
      
      >>> predict_temperature([21, 22, 20])
      21.0
      """
output:     alpha = 0.5
      smoothed_value = temperature_list[-1]
      for temp in reversed(temperature_list[:-1]):
          smoothed_value = alpha * temp + (1 - alpha) * smoothed_value
      return smoothed_value