Trying this in Ocaml and quickly gave up as it seems I needed to do lots of float<->int conversions.

This syntax is described in https://ocaml.org/manual/5.2/patterns.html#sss:pat-variable

lol waht

There is a similar comment below. The question description is indeed outdated.

Using C++ 17 for this question, attempting would result in an error of 'strcat' is undeclared.

https://prnt.sc/q4srxu

I worked around it with "#include <cstring>" manually...

Interesting stuff! Thanks. Too bad I don't know Haskell >_> yet

just a comment. am I supposed to have is_even call is_odd call is_even call.....................?
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wow it took me 100 lines -====-

great solution! I tested it out myself and added some comments to help me understand

Sample tests also expects only the boolean, instead of the array of three values specified in question and final tests.

I didn't get what you are asking for at first, partly because the sample test is down. I thought ((()])) might be expect 3 instead of 2 or 1, then it's a very different problem...>_>

I used an optimized backtracking, So it is possible. ;)
Think where the runtime grows and how to limit it

There is way to optimize a brutal force approach (with it's being a brutal force approach)
Try to think how the runtime is calculated and where you might be able to limit it

That's nice. I didn't use any particular sudoku specific algo but get to <6000ms, which I imagine is not bad. I know there is a dancing link algo which might apply to this kind of problems as well.