input: Write a function to print out an array backwards.
  def print_reverse_array(arr):
      for i in range(len(arr)-1, -1, -1): 
          print(arr[i])
  assert print_reverse_array([1, 2, 3, 4])==None
  assert print_reverse_array([5, 6, 7])==None
  assert print_reverse_array([])==None
output: def print_reverse_array(arr):
      for i in range(len(arr)-1, -1, -1): 
          print(arr[i])