A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
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.
"Do geese see God?" is a palindrome.
"A man, a plan, a canal: Panama" is a palindrome.
"race a car" is not a palindrome.