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;
	}
}