added ABC Problem

Added two implementations of the ABC problem.
You're right.
`final.len() == input.len()` like I do in the second method works too and would probably perform even better but it's probably more obscure ("why is he checking the len() here?")

I need to do it in the second case because when I start iterating from blocks, the chars are not found in the order they have in the original string... What do you think?