input: Write a function to determine if a given string s is a palindrome. A palindrome is a word, phrase, number, or other sequences of characters that reads the same forward and backward, ignoring spaces, punctuation, and capitalization. If the string is a palindrome, return True; otherwise, return False.
  assert is_palindrome("racecar")==True
  assert is_palindrome("A man, a plan, a canal, Panama")==True
  assert is_palindrome("hello")==False
  assert is_palindrome("No lemon, no melon")==True
  assert is_palindrome("")==True
output: def is_palindrome(s):
      cleaned_s = ''.join(char.lower() for char in s if char.isalnum())
      return cleaned_s == cleaned_s[::-1]