public boolean isPalindrome(String s) {
    int i = 0, j = s.length()-1;
    while (i<j){
        //to ignore characters that are not letters or digits
        while(i<j && !Character.isLetterOrDigit(s.charAt(i))) i++;
        while(i<j && !Character.isLetterOrDigit(s.charAt(j))) j--;
        if (Character.toLowerCase(s.charAt(i)) != Character.toLowerCase(s.charAt(j))){
            return false;
        }
        i++;
        j--;
    }
    return true;
}