Except that the OP specifically says that the program needs to "create lists untill no more strings can be create." Like the other requirements, it's a bit vague, but as this thread has gone well beyond the point of merely helping him with his homework assignment, it is not unreasonable for us to consider some of the ramifications of both creation and storage.

That said, a 1-for-1 hashing between a random integer and a valid sequence, as you suggest, is almost certainly the ideal solution for sequence generation. I'm sure it could be done by almost everyone that has posted in this thread so far, but doing so and posting the solution here would hand the OP an A+ on a silver platter, and frankly, I don't think he's earned it yet.