input: Write a python function to generate a random alphanumeric string of a given length.
  assert len(generateRandomString(5))==5
  assert len(generateRandomString(10))==10
  assert isinstance(generateRandomString(8), str)
output: import random
  import string
  
  def generateRandomString(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))