input: Write a python function to remove special characters from a string.
  assert remove_special_characters("ABC%@#XYZ") == "ABCXYZ"
  assert remove_special_characters("Hello, World!") == "HelloWorld"
  assert remove_special_characters("123@#456$%^") == "123456"
output: def remove_special_characters(s):
      return ''.join(e for e in s if e.isalnum())