According to wikipedia[1], the sequence goes
1, 11, 21, 1211, 111221, 312211, 13112221, 1113213211, ...
Your major problem would be finding an interviewer who knows what's an L-System
Edit: you should add the rule 111->31, so it may not be an l-system because there are infinite rules? (1111->41), etc
Or we limit it up to digit 9
That's an issue with any sequence-style problems.