Sudoku - math puzzle game

The Professional Sudoku game is coming! With four different degrees of difficulty and more game model. Sudoku is a math game, you should use 1 to 9 to fill the 9 * 9 board, to ensure each row, each column and each 3x3 small checkerboard all contain the numbers 1 to 9. The Sudoku game is not only very funny, but also can exercise your brain. Speed up your response, grow your IQ.