I thought the next number in the series from the first post would be:

1121121211

But if you were going that route then you'd still be wrong anyway;
111111121211
And then;
111111111111111211121111
111111111111111111111111111111121111111211111111
etc.

Alternatively you could have also said;
2111121211.

The 'accepted' answer (312211) is the only one that avoids ambiguity.