input: Write a function to check if a given string is a palindrome. A palindrome is a string that reads the same backward as forward. The function should return True if the string is a palindrome, otherwise False. If the input string is empty, the function should return True.
  
  
  def is_palindrome(s):
      s = s.lower()
      left = 0
      right = len(s) - 1
      
      while left < right:
          if s[left] != s[right]:
              return False
          left += 1
          right -= 1
      return True
  
  assert is_palindrome("racecar") == True
  assert is_palindrome("hello") == False
  assert is_palindrome("A man a plan a canal Panama") == True
  assert is_palindrome("") == True
  assert is_palindrome("No lemon, no melon") == True
  
output: def is_palindrome(s):
      s = ''.join(filter(str.isalnum, s)).lower()
      return s == s[::-1]