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.

 

public class Solution {
    public boolean isPalindrome(String s) {
        int len = s.length();
        if(len<=1) return true;
        s = s.toLowerCase();
        int start = 0;
        int end = len-1;
        while(start<end){
            if(!check(s.charAt(start))) start++;
            else if(!check(s.charAt(end))) end--;
            else if(s.charAt(start)==s.charAt(end)){start++; end--;}
            else return false;
        }
        return true;
    }
    public boolean check(char a){
        if(a<='z' && a>='a' || a<='9' && a>='0'){
            return true;
        }
        else return false;
    }
}