input: Write a function to check if a given string is a palindrome.
  assert is_palindrome('racecar') == True
  assert is_palindrome('hello') == False
  assert is_palindrome('madam') == True
  assert is_palindrome('step on no pets') == True
  assert is_palindrome('palindrome') == False
output: def is_palindrome(s):
      s = s.replace(" ", "").lower()
      return s == s[::-1]