input: Write a python function to print "hello world" n times.
  assert print_hello_world_n_times(3) == "hello world\nhello world\nhello world\n"
  assert print_hello_world_n_times(1) == "hello world\n"
  assert print_hello_world_n_times(0) == ""
output: def print_hello_world_n_times(n):
      return "hello world\n" * n