input: Write a function to print a right-angled triangle of stars.
  assert stars(3) == "* \n* * \n* * * \n"
  assert stars(5) == "* \n* * \n* * * \n* * * * \n* * * * * \n"
  assert stars(1) == "* \n"
output: def stars(n):
      return ''.join('* ' * (i + 1) + '\n' for i in range(n))