You start by being baited on the Linkedin job, The salary is of course very attractive (130k euros) so you know (and them as well) that there will be tens of people applying, they have to filter applicants somehow, I get it. So you send your resume on their online platform. Before you are allowed to speak to a human, they will send you a difficult but doable code challenge to do in 9 hours on your own time. You have 2 attempts (clearly written in the description), after the 1st one they give you some hints if you didn't succeed. Their requirements are very strict but nothing extraordinary if you are comfortable in C++17. I started wrapping my head around the problem for ~1 hour, then writing my tests, which took about 1 hour, and then wrote and optimized the algorithm in ~3 hour. They will not care about your tests, only about your algorithm. The challenge was actually quite nice, I enjoyed it, but didn't meet their requirements in the end. Some guys here were hurt on their ego because they didn't pass, well that's to be expected! I didn't know much about think-cell, but unfortunately some previous guys before me were right : it is a very impersonal process that doesn't care about anything else than if you can code a 100% error-free algorithm by yourself without being able to see the tests or run them. A rep from the HR team has written here on Glassdoor that the tests "give a potential candidate the possibility to learn more about the work process at think-cell". I'll let you decide if you think this is a good software development process :) I understand they have to filter applicants in an automated way, but it's a bit awkward that job advertisement practices like the LinkedIn baiting is why they have to do this filtering in the first place. Even Microsoft and other giants will let you talk to a human when you do your code challenge... Like some previous guy wrote, Prepare your environment completely before. Have a test framework ready and compile your code with Godbolt because the requirement says to use g++ 7.2.

After spending hours to do a test - they reject the application without even bothering providing feedback (and that despite the fact the test was well done and passing all the tests) - that is not a professional behavior and lacks respect for the time people invest in the process.