input: Write a function to calculate the Gini coefficient of a given dataset. If the dataset is empty then return 0.
  assert gini([]) == 0
  assert gini([2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 21]) == 0.605
output: def gini(data):
      if not data:
          return 0
      n = len(data)
      data.sort()
      total = sum(data)
      gini_sum = 0
      for i, value in enumerate(data):
          gini_sum += (2 * (i + 1) - n - 1) * value
      return gini_sum / (n * total)