In how many ways you can make a stack of N disks, such that:
Bottom disk always has radius 1
A disk can be placed on the stack if it radius is <= (maximum of all disk radii below it + 1)
You are given an infinite supply of disks with all radius.
I thought of couple of counting approach but that has lots of overlapping in the already counted solution and I am not able to get rid of it. :(