input: Write a python function to find the maximum n elements from an array.
  assert max_n_elements([2, 3, 4, 5, 1], 3) == [5, 4, 3]
  assert max_n_elements([10, 20, 15, 30, 5], 2) == [30, 20]
  assert max_n_elements([1, 2, 3, 4, 5], 1) == [5]
output: def max_n_elements(arr, n):
      return sorted(arr, reverse=True)[:n]