Add article for day 07

[Philippus]

No description provided.

[SethTisue]

Thank you @Philippus ! I had volunteered for day 7, but I saw your PR in time and at #571 I moved my volunteer slot to day 10 instead. If you plan to submit any further articles, please check the schedule first.

[Philippus]

Thank you @Philippus ! I had volunteered for day 7, but I saw your PR in time and at #571 I moved my volunteer slot to day 10 instead. If you plan to submit any further articles, please check the schedule first.

Ah, I'm sorry. I didn't know about the schedule.

[SethTisue]

@Philippus you might enjoy: (SPOILER)

instead of processing the numbers in the equation from left to right, I process them from right to left. this allows you to prune the search space a lot and not test all possible equations. for example if you've got: 12345: ..... 2 you can see that 12345 isn't divisible by 2, so you don't need to consider equations ending in * 2, and 12345 doesn't end in 2 so you don't need to consider equations ending in | 2. so you can recurse only on the possibility of addition and continue with 12343: ...

the sample input today isn't big enough for it to really matter, but the approach could be worth keeping in mind in case it becomes applicable again later in the month

(for the article, though, I think the approach you took is totally fine)