How early I can apply for US citizenship?

來源: 於 2006-07-26 14:09:31 [舊帖] [給我悄悄話] 本文已被閱讀：次

I know I can apply citizenship 5 years after getting green card. But I heard that I can apply earlier to save time.
Can somebody tell me how early I can apply? And why I can apply earlier than 5 years? Are there any difference between different states?
Whoever answer my question, thank you very much.