How to solve Sudoku

Hints on how to play Sudoku:

1. If one of the digits is already present somewhere in the grid, you should continue working on that digit, because it will be easier to fill the other rows and columns with that one.
2. It is a good idea to write down the digits that are already present in every row, column and subgrid, because you don’t have to deal with that digit anymore.
3. You should start with those subgrids, rows and columns that contain the most digits, because you have a bigger chance of solving those.
4. If a certain digit is already present in a row, column or subgrid, then it can’t appear more than once, so you can forget about it as an alternative digit in other squares. You should start the game by finding the place of those digits that can be determined without any doubt.
5. When you fill a square with the correct digit, the number will turn green, but at a certain point of the game it is possible that more digits could be correct.
   If at that certain point the digit is incorrect, it will turn red.
   It is possible that after filling the other squares, some of the digits written down previously won’t be correct anymore. In this case the number won’t change its color, you’ll have to find the incorrect digit in the grid.