Basic techniques to solve Sudoku puzzles
Checking
Starting from an empty cell we check the values that are missing from the row, column or region where it belongs. If there is only one missing number, we have the solution for that cell.
We can see in the example that checking the second row there are all the values except for the number 1, so it become the solution for the empty cell.
Cross checking
It is a variation of the checking method that takes into consideration simultaneously the values from the row, column and region where the empty cell belongs.
Starting from the marked cell, C5, we see that checking simultaneously its row, column and region, (row 5, column C, region R4), there is only the number 7 remaining, which is the solution for the cell.
Detailing more the steps, first we see that in region 4 only remains the numbers 1, 3, 4, 5 and 7, while in column C only remains 1 and 7, but on the row appears the number 1, so the only possible solution is number 7.
Scan
We scan a row, column or region to reject a number from another row, column or region, so if it only remains an empty cell, that number will be that cell solution.
In this case the number 2 appears on rows 1 and 2, so it only remains row 3. As the second region must contain the number 2 and there is only one cell left on row 3 for this region, its solution is 2
Blocked numbers
If a number must be in some cell from a row or column of a certain region, we can reject as a possible value for the rest of that row or column cells out of the region.
In the two marked cells must appear the numbers 1 and 7, because they are the last remaining on their region, so we can reject these two values from the rest of the row.
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