It's just simple luck, not cheating. I had a similar experience, at a Microsoft interview no less, a decade+ ago. I had been playing around with AI Pathfinding

It's just simple luck, not cheating. I had a similar experience, at a Microsoft interview no less, a decade+ ago. I had been playing around with AI Pathfinding algorithms for a game I was working on. The first interviewer tries to toss me a difficult question that involved navigating a complex graph. I'm sure it was supposed to be a multi-stage problem that we work through together. Instead, I walked up to the board and wrote out an A* algo in 45 seconds because I had been writing this F-ing algo over and over, troubleshooting it for days in my game. The interviewer looked awestruck, and we spent the rest of the time discussing other things. Of course, I got the job. It's just plain luck. Enjoy it when it happens.