It's nice, I had similar solution, but even now I cannot pass performance tests with it.

I'd probably wrap it in a class and refactor #go_up and #go_down, but it's working ;).

I suspect your solution is faster because it doesn't have to create additional Arrays like the each_cons in my solution does.

Most of the time I go for readability over performance unless it's a particularly critical piece of code.

I had another go at this one, what do you think?

Man, great job, but I don't understand what is the rule it follows. How did you find the pattern?

Why not (1...num)? And why do you need inject(0, &:+) instead of inject(:+)?

Neat, no iteration included :).

Can you explain why flatten is better?

I tried this in Ruby 2.2.2 and it works, when you put it into method. However it doesn't work in irb.