We need to fill in a 9x9 grid with digits so that each column, row, and 3x3 section contain the numbers between 1 to 9.
Some squares are already filled in at start. Our job is to use logic to fill in the missing digits and complete the grid.
A digit is correct when:
- Each row contains NOT more than one of the same number from 1 to 9
- Each column contains NOT more than one of the same number from 1 to 9
- Each 3x3 grid contains NOT more than one of the same number from 1 to 9
The easiest way to begin is to choose a number and try to fill in all the places in the puzzle where it must appear, according to the Sudoku rules.
When you finish dealing with the first number, choose another number.
Repeat these steps for all the numbers from 1 through 9.
As the puzzle starts to fill, you will find new places to fit the earlier numbers, so you may go back to those other numbers and place them in the grid.
At the end there will be no empty boxes left, and the puzzle will be complete.
The three red questionmarks are in places where there is a missing value.
Because the rest of the line or box is complete, it's easy to find which value must be left:
- The only number missing from the horizontal row is a 7.
- The number missing from the vertical column is a 6.
- The value missing from the box is a 5.