Last possible number technique

Strategy #3 for solving Sudoku: Last possible number

The technique of the last possible number is widely applied to solve Sudoku of all difficulties. It's a fundamental strategy that helps both beginners and experienced puzzle solvers.

The essence of this strategy is to find the "weakest cell." A weak cell is one that can only accept one value. Typically, such cells are found at the intersection of rows and columns where the greatest number of digits are already known. 

You need to systematically try each digit from 1 to 9 in the current cell. While doing this, ensure that the current row, column, and 3x3 block do not already contain that digit. If you find such a digit, then you've successfully utilized this Sudoku solving method.

However, there are cases when a cell can have multiple values. If there are only 2 possible values, it's recommended to note them down in the cell. If there are more than 2 candidates, you should look for another "weak cell."

In the image above, you can see the technique of the last possible digit in action. We'll consider the cell highlighted in dark blue. In the column it belongs to (green rectangle), the following digits are already present: 2, 3, 4, 6, 9. In the row (orange rectangle), there are unique digits 7, 8. In the 3x3 block (pink square), there's a 5 which doesn't appear in the row or column. If we combine the unique digits from the row, column, and the 3x3 square, we get: 2, 3, 4, 5, 6, 7, 8, 9. It's easy to guess that the only missing digit is 1, so we'll place it in the dark blue cell.

Like the strategy? We recommend practicing it on a medium 9x9 Sudoku. You can use all previously known Sudoku solving techniques on it.