I think it must have seen the solution somewhere already on the web. I find it extremely hard to believe that such a general purpose chatbot would just be able to solve programming problems. Deepmind had a paper [1] on solving programming problems a couple of months ago and they had to apply quite specialized heuristics in order to solve these problems. Obviously ChatGPT does nothing of the sorts and it just seems extremely unrealistic that it would be capable of outperforming previous work like that.
[1] https://news.ycombinator.com/item?id=30179549