Description

This is the simplest strategy which is used at the start of a puzzle. The idea is to find any cell which can only be one value based on all the other cells on the board and the Sudoku rules.

Reasoning

These strategies do not require pencil marks to be entered, but at first you might find it helpful to keep track of cells you've already looked at.

Algorithms

Box Search

1. Start with the top left box and look to see where each digit from 1 to 9 can go, and if it can only be in one cell, you found the solution for that cell.
2. Repeat for every box.

Row Search

1. Start with the top most row and look to see where each digit from 1 to 9 can go, and if it can only be in one cell, you found the solution for that cell.
2. Repeat for every row.

Column Search

1. Start with the left most column and look to see where each digit from 1 to 9 can go, and if it can only be in one cell, you found the solution for that cell.
2. Repeat for every column.

Once you have completed all these steps, you start again from the top, until you find no solutions to any cell during a complete cycle. At this point you know that there are no more singles to be found and you move on to the next strategy.

Digit Search

Another strategy to find singles is to take one digit at a time and figure out where it could be on the entire puzzle board.

1. Figure out which digit has the most givens on the board and start with it.
2. Figure out where in each box the digit might be able to go by looking at the rows and columns that interserct that box. If it can only go in one cell of that box, then you have found the solution for that cell.
3. Figure out where in each row the digit might be able to go. It is can only go in one cell of a given row, then that is the value of that cell.
4. Repeat with every column for the currently selected digit.
5. When you finished going through all boxes, rows and columns for the current digit, select the next digit that has the most givens and start the process again until you finish all the digits and you have found no more singles during a complete cycle of all 9 digits.

The Sudoku Tutor Lock Mode was designed to help with the Digit Search strategy. It allows you to Lock in a digit and then every cell you tap, will get that value. This dramatically speeds up the solving early on.

To further speed up the search for singles, you can use the Possible grid mode, which shows you where a particular selected digit can go, by highlighting these cells in green.

Naked Single Search

One more strategy that can be done without pencils marks, is to look at every cell and figure out if only one value can go in it, which would be the solution. This strategy is called Naked Single and is easier to do once the pencil marks have been put in.

Practice Puzzles

If you tap on the following links on an iOS device which has the Sudoku Tutor app installed, it will launch the app and open the practice puzzle. Tap hint once the puzzle is open to see the strategy in action.

Sample Puzzle 1
Sample Puzzle 2

Next Step

Back to Sudoku Solvers or continue to next algorithm Pencil Marks