How to Play Evil Sudoku

Evil Sudoku is our highest difficulty level. Like any Sudoku puzzle, the goal is to complete the 9x9 grid by filling in the missing numbers so that each row, column, and 3x3 block contain the numbers 1 through 9 exactly once.

However, you have fewer givens, so you must use advanced Sudoku strategies like X-wing, XYZ-wing, swordfish, forcing chains, and almost locked sets to eliminate potential candidates and deduce answers.

Evil Sudoku Strategies

The Evil level takes patience and persistence to solve. You’ll need to use advanced techniques and careful observation to fill in the cells and eliminate candidates. These Sudoku solving techniques, and the ones featured in our advanced Sudoku guide, will help you master challenging puzzles.

Forcing Chains

The forcing chains technique uses “if, then” logic by identifying a cell with two candidates and modeling what chain reaction will happen if one candidate is the answer or the other candidate is the answer. In doing so, you’ll find contradictions or confirmations, which help you determine answers or eliminations.

This solution is not guessing because it leaves you with a logical outcome. For example, in the scenario below you’ll find that H1 has the same answer regardless of whether B1 is 8 or 9. This confirms that H1 is 4.

For example:

• If B1 = 9, then H1 = 4 and H5 = 9.
• If B1 = 8, then B7 = 2, B4 = 6, C5 = 2, H5 = 9, and H1 = 4.

Since the chain of events starting at B1 confirms that H1 = 4 in both scenarios, you know that 4 must be the answer to H1.

Almost Locked Sets (ALS)

A locked set is a group of cells within a unit where the number of candidates matches the number of cells they occupy. For example, if two potential candidates are confined to two cells, also known as a naked pair, that set of two is a locked set.

An almost locked set (ALS) occurs when you find a group of cells within a unit that contains one more candidate than the number of cells. For example, if three potential candidates (1, 6, 7) are spread across two cells—(1, 6) in one and (6, 7) in another—that forms an almost locked set.

When two almost locked sets share a restricted common candidate, it forms a continuous chain within the group of cells, which can lead to eliminations. Remember, for a pair of almost locked sets to be helpful, they must meet the following criteria:

• All cells in each set must be able to “see” (be in the same row, column, or box) another cell in the almost locked set.
• There must be a restricted common candidate, which is a candidate that appears in both almost locked sets once, but can only be the answer to one of the sets.
• There must be a common candidate found in both sets that is not the restricted common candidate.

If all these things are true, then any candidate on the grid that can “see” all the common candidates in both ALS groups can be removed.

In the example below, C3 and C6 make up the first ALS and G1 and G6 make up the second ALS. They are connected by the restricted common candidate 6, which appears in C6 and G6, but can only be the answer for one ALS or the other, not both. Cells C3, G1, and G6 have the common candidate 4, so any 4 candidate outside of the ALS groups that has a can see those three cells can be eliminated. In this case, the 4 candidate in G3 is eliminated.

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