Long time no see
"Long time no see" a common greeting to say that we haven't met for a long time. It's a greeting, so the respond should be the same with a little different intonation.
It is a grammatically comical greeting with English words organized by Chinese grammar. In Chinese there is an everyday greeting "Hao jiu mei jian" which corresponds with "long time no see", literally word by word. It is said that "Long time no see" was first used by Chinese-Americans.