Most of those code tests are scam. It's a psychological tool used by companies to improve their bargaining position vis a vis employees. It's called the IKEA-effect -- people like IKEA furniture more because they had to assemble it themselves. Programmers become more attracted to job opportunities if they have to work for it.

I've been asked to write constraint solvers for hexagonal Graeco-Latin squares which I did complete. It took me days, I got an optimal solution and was the only candidate able to do so. Still didn't get the job.

My current strategy is to only agree to solve puzzles if the recruiter can tell me how many of my potential coworkers solved what puzzles when they were hired. If it turns out most of them solved very challenging ai or dynamic programming problems, then I could jump through almost any hoops to get the job. Because working with people smarter then you is just invaluable.

If they aren't able to divulge such information, or most of the employees didn't solve any puzzles at all, I would be weary.