Heh - I forgot the trick where if you set a default value in the function signature, the instance persists.

Thanks for the pointer

How big are the numbers you are testing locally? In the tests inputs go up to 1000. When I run your memoized function (after manually increasing the recursion limit), it takes a good 20 seconds or so. You need a better algorithm (Hint: memoize results between function calls too, not just for a single function call)

Note: Using Python

Testing my algorithm offline, I can complete a 100000 test run in 1.15 seconds. In the codewars environment, this times out after 12 seconds, meaning that I cannot submit my solution.

Annoying...