input: Write a function to determine if a given string is a palindrome. If it is, return True, otherwise return False. A palindrome is a word, phrase, number, or other sequence of characters that reads the same forward and backward.
  assert is_palindrome("racecar")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("12321")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      s = ''.join(e for e in s if e.isalnum()).lower()
      return s == s[::-1]