class UserComparator implements Comparator<User> {
    public int compare(User u1, User u2) {
        if (u1.number.charAt(0) == u2.number.charAt(0)) {
            // 如果两人的号都是A开头或者都是V开头,比较号的大小:            
            if (u1.number.length() == u2.number.length()) {
                return u1.number.compareTo(u2.number);
            }
            return u1.number.length() - u2.number.length();
        }
        if (u1.number.charAt(0) == 'V') {
            // u1的号码是V开头,优先级高:
            return -1;
        } else {
            return 1;
        }
    }
}