# How to Solve Sudoku When Stuck

Sudoku puzzles can be challenging for beginners, but there are several strategies that will help you conquer even the toughest puzzles.

### Utilizing Pencil Marks

A straightforward and successful strategy is to use pencil marks to store which candidates remain viable in each cell. Doing this allows you to identify areas where all possibilities have been eliminated, leaving only one number that fits.

This method can be employed both manually and electronically, with the benefit that you can eliminate any invalid candidates. It also serves to ensure the correct number of cells are filled in.

### Trial and Error (T&E)

A common strategy to find the correct solution involves trying different values of numbers in each square, column, and row. This process can be repeated as many times as necessary until you find it.

Once you’ve tried different values, you should be able to distinguish the patterns created. This is particularly helpful when trying to figure out how to allocate a particular number to a row, column, or box.

### Almost-Locked Sets

Almost-locked sets are a type of bent naked subset and an invaluable aid when solving Sudoku puzzles. They appear frequently throughout the grid and offer many solutions.

Groups of n cells where n+1 are possible and n+2 are impossible are known as “unique”. They’re commonly encountered in early levels of the game, making them beneficial for new players by providing some assistance with strategy decisions.

They’re easy to recognize, making them the go-to strategy for Sudoku players who want a straightforward strategy that’s easy to use and frequently encountered on easy levels.

### Swordfish and X-Wing

A more advanced version of a hidden single, an X-wing occurs when groups of cells have only n+1 possible candidates. This occurs because there must be at least two values outside the group that cannot be filled in by existing values.

In this instance, since 1 is not included in any of the n+1 groups, there are no candidates for inclusion. This can be done with any number, but the smallest possible value is 6.

Each group must contain a value that is distinct from the others. This could be an unaccounted-for digit or an empty cell.

An almost-locked set is one in which n+2 possibilities exist in both groups but not all of them are in the other. While this type of set can be highly efficient in producing eliminations, it may not always be obvious.

Sudoku Assistant uses almost-locked sets in the game to generate more eliminations than when they aren’t present. It does this by checking for mutually-linked pairs of almost-locked subsets (bent naked subsets) and eliminating any weak links that are discovered.