input: Write a python function to normalize a list of numbers.
  assert normalize([1, 2, 3, 4, 5]) == [0.0, 0.25, 0.5, 0.75, 1.0]
  assert normalize([10, 20, 30]) == [0.0, 0.5, 1.0]
  assert normalize([5, 7, 9, 11]) == [0.0, 0.3333333333333333, 0.6666666666666666, 1.0]
output: def normalize(numbers):
      if not numbers:
          return []
      min_val = min(numbers)
      max_val = max(numbers)
      if min_val == max_val:
          return [0.0] * len(numbers)
      return [(num - min_val) / (max_val - min_val) for num in numbers]