This is a nice follow-up to your previous post, especially because DepthFirstSearch doesn't lead to a timely solution. However, it's well-known that Sudoku is a good use case for Donald Knuth's DancingLinksX algorithm, so it's also possible to compare times with FindExactCover. Using the MRV-heuristic, we can achieve a $2\times$ speed-up solving the puzzle as an exact cover problem. There is a deb
