[Leetcode] Valid Palindrome

Problem

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.

Algorithm

s.toLowerCase() first to ignore cases. 

Write a helping method isAlphaNumeric to reduce code. 

Write two helping methods to find the next and previous alphanumeric character's index, which is easier to read. 

Code