input: Write a python function to remove non-alphanumeric characters from a string.
  assert remove_nonalphanum("Thi$ i$ a $tring with $ymbol$!") == "ThisisaStringwithymbols"
  assert remove_nonalphanum("Hello, World!") == "HelloWorld"
  assert remove_nonalphanum("123@456#789") == "123456789"
output: def remove_nonalphanum(s):
      return ''.join(c for c in s if c.isalnum())