class Solution {

    public boolean is_alphanumeric(char c) {
        if (c >= 'a' && c <= 'z') return true;
        if (c >= 'A' && c <= 'Z') return true;
        if (c >= '0' && c <= '9') return true;

        return false;
    }

    public boolean isPalindrome(String s) {

        int i_left = 0;
        int i_right = s.length() - 1;

        while (true) {

            Character c_left = s.charAt(i_left);
            while (is_alphanumeric(c_left) == false) {
                i_left++;
                if (i_left > i_right) break;
                c_left = s.charAt(i_left);
            }

            if (i_left > i_right) break;

            Character c_right = s.charAt(i_right);
            while (is_alphanumeric(c_right) == false) {
                i_right--;
                if (i_left > i_right) break;
                c_right = s.charAt(i_right);
            }

            if (i_left > i_right) break;

            if (Character.toLowerCase(c_left) != Character.toLowerCase(c_right)) {
                return false;
            }

            i_left++;
            i_right--;
            if (i_left > i_right) break;

        }

        return true;

    }
}