In the first exam I wrote, while doing a Postdoc at a university, I made a question with a really neat algorithm I'd thought of. I spent a lot of time making it

In the first exam I wrote, while doing a Postdoc at a university, I made a question with a really neat algorithm I'd thought of. I spent a lot of time making it super clean, ramping up to the algorithm using hints, etc. I was very happy when I read the first exam reply and the student had correctly solved the entire question! But then the second student had the same solution, and the third, and the fourth... Turns out somebody had written the exact same algorithm on stackoverflow, and I hadn't noticed. The students were allowed to use the internet, but they were also required to say what material they had used. Only one student actually listed the stackoverflow answer. My first instinct was to report all the students for cheating. But then, there were so many of them that I couldn't be bothered. I just changed the weighting of the question to 0.