📚 Learn Sudoku

📋 Basic Rules

Sudoku is played on a 9×9 grid, divided into nine 3×3 boxes.

• Each row must contain the digits 1‑9 exactly once.
• Each column must contain the digits 1‑9 exactly once.
• Each 3×3 box must contain the digits 1‑9 exactly once.

A well‑formed puzzle has only one solution, and you never need to guess.

🟢 Basic Strategies

• Naked Single

  A cell has only one possible number because all other numbers already appear in its row, column, or box. Just place it!

• Hidden Single

  A number can only go in one cell within a row, column, or box, even if that cell has other candidates.

🟡 Intermediate Techniques

• Naked Pair

  Two cells in the same unit (row/column/box) share exactly the same two candidates. Those candidates can be eliminated from all other cells in that unit.

  Example: if cells A and B both have candidates {4,7}, no other cell in that row can contain 4 or 7.

• Hidden Pair

  Two numbers appear exactly in the same two cells of a unit, hidden among other candidates. All other candidates can be removed from those two cells, leaving only the pair.

• Pointing Pair / Box‑Line Reduction

  If a candidate appears only in one row or column within a box, it can be eliminated from the rest of that row/column outside the box.

🔴 Advanced Techniques

• X‑Wing

  When a candidate appears exactly twice in two rows, and those occurrences align in the same two columns, you can eliminate that candidate from other cells in those columns (and vice‑versa with rows).

• Swordfish

  A 3‑row / 3‑column extension of X‑Wing. If a candidate appears in exactly three rows and aligns in three columns, it can be removed from those columns in other rows.

• XY‑Wing / Y‑Wing

  A chain of three cells with exactly two candidates each, connected in a way that forces a common candidate to be eliminated from cells that see both ends.