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]) == [40, 30, 20, 10]
  assert reverse_array([5, 5, 5, 5]) == [5, 5, 5, 5]
output: def reverse_array(arr):
      return arr[::-1]