Point $$B$$ is on $$\overline{AC}$$ with $$AB = 9$$ and $$BC = 21.$$ Point $$D$$ is not on $$\overline{AC}$$ so that $$AD = CD,$$ and $$AD$$ and $$BD$$ are integers. Let $$s$$ be the sum of all possible perimeters of $$\triangle ACD.$$ Find $$s.$$

(第二十一届AIME1 2003 第7题)