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 take a list of the past three temperatures and return the predicted temperature.
  assert predict_temperature([21, 22, 20])==20.5
  assert predict_temperature([19, 20, 18])==18.5
  assert predict_temperature([23, 24, 22])==22.5
  assert predict_temperature([20, 21, 19])==19.5
  assert predict_temperature([25, 26, 25])==25.0
output: def predict_temperature(temps):
      alpha = 0.5
      return alpha * temps[2] + (1 - alpha) * (alpha * temps[1] + (1 - alpha) * temps[0])