Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.
Have you consider that the string might be empty? This is a good question to ask during an interview.
For the purpose of this problem, we define empty string as valid palindrome.
O(n) time without extra memory.