【后缀】保留最后点 输出后缀

   public static void main(String[] args) {
        String input = "com.2022.2326.img.jpeg";
        String output = lastDot(input);
        System.out.println(output);
        // 输出: .jpeg
    }

    public static String  lastDot(String str) {
        int last = str.lastIndexOf('.');
        if (last != -1) {
            return str.substring(last);
        }
        return str;
    }