input: Write a python function to print "Hello World!" a specified number of times.
  assert hello_world_n_times(3) == "Hello World!\nHello World!\nHello World!\n"
  assert hello_world_n_times(1) == "Hello World!\n"
  assert hello_world_n_times(5) == "Hello World!\nHello World!\nHello World!\nHello World!\nHello World!\n"
output: def hello_world_n_times(n):
      return "Hello World!\n" * n