Valid Palindrome

Write `is_palindrome(s)` that returns `True` when the string reads the same forwards and backwards, ignoring case and skipping anything that is not a letter or digit. is_palindrome("A man, a plan, a canal: Panama") -> True is_palindrome("race a car") -> False Building a cleaned copy and comparing it to its reverse is correct and costs O(n) extra space. The version worth writing walks a pointer in from each end and skips non-alphanumeric characters as it goes.