I want to point out that, just like in real life, its the inclusion of that "++" which complicates things. Without them, it would be straightforward and elegant, having only 1 logical solution.

Makes you wonder why people keep insisting on having the "++" around. Why not ditch them and go back to basics.

C++ programmers always let their friends access their private members, so ...

Judging from how it feels to write the language, it's definitely number 2.