[misja111]

> This is why it can be considered memoization: each recursive call builds on the "stored" finished result of the previous call.

Ok, but isn't this then also the case for non-tail recursive calls? And ordinary recursive call builds just as well on the finished result of the previous call and no result is really "stored" in either case.

[ohkaiby]

No, if you look at vanderZwan's explanation for the non-tail recursive call, you'll see that it doesn't build on a finished result of the previous call.

> You have to recurse until you reach n == 0, and only then does the whole sum "collapse".