input: Write a python function to generate a random string of a given length.
  assert len(generateRandomString(8)) == 8
  assert len(generateRandomString(12)) == 12
  assert len(generateRandomString(0)) == 0
output: import random
  import string
  
  def generateRandomString(length):
      return ''.join(random.choices(string.ascii_letters + string.digits, k=length))