input: Write a function to determine if a given string is a palindrome. A palindrome is a string that reads the same backward as forward. If the string is a palindrome, return True; otherwise, return False.
  assert is_palindrome("racecar")==True
  assert is_palindrome("level")==True
  assert is_palindrome("python")==False
  assert is_palindrome("A man a plan a canal Panama")==False
  assert is_palindrome("")==True
output: def is_palindrome(s):
      s = s.replace(" ", "").lower()
      return s == s[::-1]