input: Write a function to reverse an array.
  assert reverseArray([1, 2, 3, 4, 5]) == [5, 4, 3, 2, 1]
  assert reverseArray([10, 20, 30, 40]) == [40, 30, 20, 10]
  assert reverseArray(['a', 'b', 'c']) == ['c', 'b', 'a']
output: def reverseArray(arr):
      return arr[::-1]