Fill a 9x9 grid with digits so that each column, each row, and each of the
nine 3x3 sub-grids contains all of the digits from 1 to 9 once.
Select a number on the toolbar below, and then tap the cell on the board to place your selection.
Select Pencil icon to toggle between selection/mark modes. From the main menu (gear icon in the
top right corner) select hints to update valid options for a cell or to suggest next move.