How to Play Expert Sudoku

Expert Sudoku provides an ultra-challenging level of puzzle solving. Although it is a typical 9x9 grid, you have far fewer givens and must use expert-level strategies to solve the puzzle by placing numbers 1 through 9 in each cell without repeating them in any column, row, or 3x3 box.

Expert Sudoku Strategies

While you can use any strategy to solve Sudoku puzzles, beginner-level strategies, such as crosshatching, won’t help much with expert-level puzzles because you don’t have many givens. You must rely on advanced strategies to solve these difficult puzzles.

XY-Wing

This technique, also called the y-wing, helps you eliminate candidates from other cells. For this technique, three things must be true:

• Three cells each have two of the same three candidate numbers and no other candidates. For example, in cells A2, A7, and C2, each cell contains two of the same three candidate numbers, 3, 5, and 9. These cells can be connected in a shape that resembles the letter y.
• Only one cell shares a unit with each of the other cells, called the pivot cell. In this example, A2 is the only cell that can overlap with units of the other two because it’s in column A (shared with A7) and row 2 (shared with C2).
• The two cells share one number with the pivot cell.

So in this example, if the answer to A2 is 3, then C2 is 9. If the answer to A2 is 5, then A7 is 9. In either case, one of the cells must have the answer 9. So you can eliminate 9 as a candidate in any cells that share a unit with A7 and C2 (the cells other than the pivot cell). In this case, the 9 candidates in A1 and C7 can be eliminated because A1 shares a column with A7 and a 3x3 block with C2 and C7 shares a row with A7 and column with C2.

XYZ-Wing

This technique follows the same parameters as the xy-wing, except that the pivot cell contains all three candidate numbers. For example, A2 now has a 9 added as a candidate. Now you can only eliminate 9 candidates in cells that share a unit with all three cells. In this case only A1 can have a 9 eliminated as a candidate because it shares a 3x3 block with A2 and C2 and a column with A7.

Unique Rectangle

This technique relies on the idea that every Sudoku puzzle has a unique solution, so to work this strategy, find the same two candidates in exactly two rows, two columns, and two blocks. In this example, 6 and 8 are candidates in cells D2, D5, F2, and F5, which are in two rows (2 and 5), two columns (D and F), and the top two middle 3x3 blocks. Note, however, that you’ll find at least one other candidate in one of the cells. In this case, a 3 in D2.

The problem is that, if the extraneous candidate (3) were marked out, then 6 and 8 in this pattern results in two solutions. So the answer to D2 and F5 is 6 and the answer to D5 and F2 is 8—or the opposite is also true, and the answer to D2 and F5 is 8 and the answer to D5 and F2 is 6. But because a puzzle cannot have two solutions, you know that 3 must be the answer in D2.

Expert-level Sudoku puzzles offer a difficult, but rewarding solving experience. However, if you’re looking for something slightly easier, try a Hard Sudoku puzzle to sharpen your skills. And for an even greater test, put your problem-solving to work with Evil Sudoku.