Please note I'm new to all this - so can you explain it simply please. Really appreciate it

I'm trying to prove that the set of all finite and countably infinite sequences over {0,1} is not countable (I think).

I have tried using the Diagonalisation technique but i'm a bit confused. I know that the set of all finite length strings is countably infinite and using the Diagonalisation technique to construct a language we can proof by contradiction that it is not countable.

Any suggestions on how to complete it or where to start (if i'm thinking about it in the wrong way)?

Thanks in advance