leetcode--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.

public class Solution {    public boolean isPalindrome(String s) {        int low = 0;int high = s.length()-1;while(low<high){if(!check(s.charAt(low))){low++;continue;}else if(!check(s.charAt(high))){high--;continue;}else{if(s.charAt(low)!=s.charAt(high)&&s.charAt(low)-s.charAt(high)!=32&& s.charAt(high)-s.charAt(low)!=32){return false;}else{low++;high--;}}}return true;    }        public boolean check(char c){if(c>='a'&&c<='z' || c>='A'&&c<='Z' || c>='0'&&c<='9')return true;return false;}}