Valid Palindrome

Given a string, determine if it is a palindrome, considering only
alphanumeric characters and ignoring cases.

For example,
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.

Note:
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.