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 {
   
    boolean isAlpha(Character a){
        return ('A' <= a && a <= 'Z') ||
        ('0' <= a && a <= '9');
    }
    public boolean isPalindrome(String s) {
        // Start typing your Java solution below
        // DO NOT write main() function
        s = s.toUpperCase();
        int i = 0;
        int j = s.length()-1;
       
        while(i<j){
            if(!isAlpha(s.charAt(i))){ i++; continue;}
            if(!isAlpha(s.charAt(j))){ j--; continue;}
            if(s.charAt(i) == s.charAt(j)){
                i++; j--;
            }
            else return false;
        }
        return true;
    }
}