ARC014C

https://gyazo.com/913d5c6fe52dfe68b9aa345fe3cb9899

Thoughts.

It's stacked, so you can't just remember the colors on the edges, because after they disappear, the colors inside appear.

There are up to 50 balls and 2^50 options.

Whether or not it is always okay to erase when you can.

It's easy if you can prove you can turn it off.

If you can't erase it, you can always erase it next time by putting it in such a way that the same color as the next color among the three colors on the left, right and current color is exposed.

This way, if the length is 2, the next is 1 or 3, and that 3 is always 2 next

Solve the short sequence by force and see if it looks OK.

I think I can make it about 2^21.

Official Explanation

The above procedure can maintain less than 3 pieces at all times.

No need to go through a complicated state because it can maintain it until the end of the station.

The logic is: [Optimal solution because it matches the lower boundary.

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