input: Write a function to generate a random password of a given length using letters and digits.
  assert len(randomPassword(8)) == 8
  assert len(randomPassword(12)) == 12
  assert len(randomPassword(0)) == 0
output: import random
  import string
  
  def randomPassword(length):
      characters = string.ascii_letters + string.digits
      return ''.join(random.choice(characters) for _ in range(length))