Sudoku

The game field is a 9×9 square divided into 3×3 small squares. Thus, the entire playing field consists of 81 cells. At the beginning of the game in some cells there are numbers (from 1 to 9), called tips. The number of tips depends on the level of difficulty chosen by the player. The player is required to fill the empty cells with numbers (from 1 to 9) so that in each row, in each column and in each small square, each digit occurs only once.