A very popular Japanese game played on 9×9 grid, which is further divided into 9, 3×3 squares, the object of the game is to fill in the
numbers 1-9 in each of the 81 boxes, without repeating a number in any
horizontal row, vertical column, or 3×3 square. This single algorithm has been used by variant developers and have released their own modded version which equally interests you. An extensive roundup has been written by Just Another Mobile Monday which is worth the read. Trust me, the game is really addictive!