input: Write a function that lists the contents of a specified directory. If the directory is not specified, list the contents of the current directory. The function should return the list of contents.
  
  def list_directory_contents(directory: str = None):
      if directory is not None:
          return os.listdir(directory)
      else:
          return os.listdir()
  
  assert list_directory_contents('/path/to/directory')==['file1.txt', 'file2.txt', 'directory1']
  assert list_directory_contents() == ['file3.txt', 'file4.txt', 'directory2']
  assert list_directory_contents('/non/existing/path')==FileNotFoundError
  assert list_directory_contents(None) == os.listdir()
output: 
  def list_directory_contents(directory: str = None):
      if directory is not None:
          return os.listdir(directory)
      else:
          return os.listdir()
  