How to Solve Hard Sudoku: Advanced Techniques
Once basic scanning stops working, the hardest puzzles demand advanced Sudoku techniques. This guide covers the strategies you need to crack SodokuDX's Hard and Evil tiers.
Naked & Hidden Pairs
When two cells in a unit share the exact same two candidates, those digits are locked to those cells — eliminate them from every other cell in the unit. Hidden pairs work in reverse: two candidates that only appear in two cells of a unit can have all other candidates stripped from those cells.
Pointing Pairs & Box-Line Reduction
If a candidate within a box is confined to a single row or column, it can be removed from the rest of that row or column outside the box. The reverse — candidates in a line confined to one box — clears the box.
X-Wing
An X-Wing forms when a candidate appears in exactly two cells of two different rows, and those cells line up in the same two columns. The candidate can then be eliminated from those columns elsewhere. The same pattern works with rows and columns swapped.
Swordfish
A Swordfish extends the X-Wing to three rows and three columns. When a candidate is confined to the same three columns across three rows, remove it from those columns in all other rows.
Forcing Chains
For the toughest Evil puzzles, forcing chains test a candidate: if assuming a value in one cell forces a contradiction down a chain of implications, that candidate can be eliminated. This is the technique that separates expert solvers from the rest.