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 accept a list of three temperatures and return the predicted temperature. If the input list has fewer than three temperatures, the function should return None.
  assert predict_temperature([21, 22, 20])==21.0
  assert predict_temperature([25, 24, 26])==25.5
  assert predict_temperature([20, 21, 22])==21.5
  assert predict_temperature([15, 14, 16])==15.5
  assert predict_temperature([23, 25, 24])==24.5
output: def predict_temperature(temps):
      if len(temps) < 3:
          return None
      alpha = 0.5
      return alpha * temps[2] + (1 - alpha) * (alpha * temps[1] + (1 - alpha) * temps[0])