Making the cache private is a bizzare "extra challenge". I can't see how that provides any better functionality/encapsulation, if anything it just makes it harder to understand.

The calls to fibonacci(n-1) and fibonacci(n-2) will store the value in memo, no reason to do this multiple times.

n is either fib(n-1) and fib(n-2)

In this solution, only fibonacci(n) is stored in the memo. But since fibonacci(n - 1) and fibonacci(n - 2) are called, they can also be stored in the memo, so if the next test is fibonacci(n - 2) for example, the result will already be in the memo.

which is exactly what is done here... Or are you talking about the base cases?

You should store the fibonacci(n - 1) in memo[n - 1] and the fibonacci(n - 2) in memo[n - 2]...

yes