I have developed an Android Sudoku App for an academic purpose. https://play.google.com/store/apps/details?id=com.yicheng.sudoku
We hope to predict the difficulty according to the patterns of a Sudoku puzzle.
So, we need to collect enough puzzles with the records from different players.
Details of the project can be found from https://lab.yichengtech.tw/research_sudoku_en.php.
