input: Write a python function to reverse an array.
  assert reverse_array([1, 2, 3, 4, 5]) == [5, 4, 3, 2, 1]
  assert reverse_array([10, 20, 30, 40, 50]) == [50, 40, 30, 20, 10]
  assert reverse_array([7, 8, 9]) == [9, 8, 7]
output: def reverse_array(arr):
      return arr[::-1]