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 given temperature list is empty or contains less than three values, return None.
  assert predict_temperature([21, 22, 20]) == 21.0
  assert predict_temperature([25, 27, 26]) == 26.5
  assert predict_temperature([18, 19, 17]) == 17.5
  assert predict_temperature([]) == None
  assert predict_temperature([30, 31]) == None
output: def predict_temperature(temps):
      if len(temps) < 3:
          return None
      alpha = 0.5
      return alpha * temps[-1] + (1 - alpha) * (alpha * temps[-2] + (1 - alpha) * temps[-3])