Jump 1 or 2 steps, Climbing Stairs

"12", it could be decoded as "AB" (1 2) or "L" (12).

It is just like jumping 1 step A B or 2 steps 12.

But, there are some restrictions.


"31" can only decode as "C" (3) and "A" (1)

So, what we need to do is to add something to the Climbing Stairs, do not climb the invalid stairs.