Ultimate Sudoku

A puzzle game from Japan that has conquered the whole world.
Fill in the empty cells with numbers from 1 to 9. In each row, column, or 3×3 block, each number can occur only once.

3 different game modes:
Cell first: First click on the cell you want to fill in, and then select a number.
Number First: first click on the number, and then on the cell in which you want to enter this number.
Note Mode: Write down the option in an empty space using the keyboard or the number buttons.