input: Write a python function to remove non-alphanumeric characters from a string and return the cleaned string.
  assert clean_string("Hello, World!") == "HelloWorld"
  assert clean_string("123@abc#") == "123abc"
  assert clean_string("No changes!") == "Nochanges"
output: def clean_string(s):
      return ''.join(c for c in s if c.isalnum())