Best-Sudoku

Traditional classic digital games
Game settings:
There are 9*9 number blocks in game, and also can be seen as 9 small area of 3*3.
Game goal:
 Fill 1~9 in each row, each column, each 3*3's area, and the numbers in each block do not conflict with the other numbers in the row, column, and area of the block.
Game basic operation:
 Select a blank position, fill in the number, or fill in an flag sign (the flag sign is used for your reference when you are solving the problem, such as possible answer of the block).
Prompt function: 
You can get the correct number in a blank position by the prompt function. If you are confused about the possible answer of a block, you can solve it by prompting.
Stages:
The game has prepared hundreds of classic stages, and users are free to choose. The program will remember the game status of each of the stage. Even if you do not finish the stage, you can still continue the stage when you open the stage next time.