class Solution {
    public boolean isPalindrome(String s) {
        // convert to lower cases
        s = s.toLowerCase();

        Integer i_left = -1;
        Integer i_right = s.length();

        while (true) {

            Character c_left = null;
            Character c_right = null;

            // move i_left, i_right to the meaning position
            while (true) {
                i_left++;
                if (i_left == s.length()) break;
                if (is_character(s.charAt(i_left))) {
                    c_left = s.charAt(i_left);
                    break;
                }
            }

            while (true) {
                i_right--;
                if (i_right < 0) break;
                if (is_character(s.charAt(i_right))) {
                    c_right = s.charAt(i_right);
                    break;
                }
            }

            if (i_left > i_right) break;

            if (c_left != c_right) {
                // wrong
                return false;
            }

        }

        return true;

    }

    public boolean is_character(char c) {
        if (c >= 'a' && c <= 'z') {
            return true;
        }else if (c >= '0' && c <= '9') {
            return true;
        }

        return false;
    }

}