Define a $$good~word$$ as a sequence of letters that consists only of the letters $$A$$, $$B$$, and $$C$$ - some of these letters may not appear in the sequence - and in which $$A$$ is never immediately followed by $$B$$, $$B$$ is never immediately followed by $$C$$, and $$C$$ is never immediately followed by $$A$$. How many seven-letter good words are there?

(第二十一届AIME2 2003 第3题)