Sudoku

Sudoku is the world's most popular logic puzzle. The player must fill a 9x9 grid with the digits 1 through 9 so that each row, each column, and each of the nine 3x3 sub-grids (called "boxes" or "blocks") contains every digit exactly once. The puzzle starts with some cells pre-filled as clues, and the challenge is to deduce the rest using logic alone — no guessing required in a well-constructed puzzle.

Basic Rules

1. The grid is divided into 9 rows, 9 columns, and 9 boxes of 3x3 cells.
2. Each row must contain the numbers 1-9 with no repetition.
3. Each column must contain the numbers 1-9 with no repetition.
4. Each 3x3 box must contain the numbers 1-9 with no repetition.
5. The puzzle has a unique solution.

Strategies

• Naked Single: If a cell has only one possible candidate after eliminating numbers already present in its row, column, and box, that candidate is the answer.
• Hidden Single: If a number can only go in one cell within a row, column, or box, it must go there, even if that cell has other candidates.
• Naked Pair/Triple: If two (or three) cells in a group share exactly two (or three) candidates, those candidates can be eliminated from other cells in the same group.
• Pointing Pairs: If a candidate within a box is confined to a single row or column, it can be eliminated from that row or column outside the box.
• Box/Line Reduction: If a candidate in a row or column is confined to a single box, it can be eliminated from the rest of that box.
• X-Wing: When a candidate appears in exactly two cells in each of two rows, and those cells share the same two columns, the candidate can be eliminated from those columns in other rows.
• Swordfish: An extension of X-Wing to three rows and three columns.

Difficulty Levels

• Easy: Can be solved using only naked and hidden singles.
• Medium: Requires naked pairs/triples and pointing pairs.
• Hard: Requires X-Wings, Swordfish, and other advanced techniques.
• Expert: May require chains, coloring, or trial-and-error in extreme cases.

The roots of Sudoku trace back to the 18th century, when Swiss mathematician Leonhard Euler developed the concept of "Latin squares" — grids where each symbol appears exactly once in every row and column.

The modern Sudoku puzzle was first published in 1979 by Dell Magazines in the United States under the name "Number Place." It was designed by Howard Garns, a retired architect and freelance puzzle constructor from Indiana. Garns added the 3x3 box constraint to Euler's Latin squares, creating the puzzle we know today. Sadly, Garns passed away in 1989 without knowing the global phenomenon his creation would become.

The puzzle was introduced to Japan in 1984 by the publisher Nikoli, who gave it the name "Sudoku" — a contraction of the Japanese phrase "suji wa dokushin ni kagiru" (the digits must remain single). Nikoli also established two important design principles: the pattern of given numbers should be rotationally symmetric, and no more than 30 clues should be provided.

Sudoku remained a Japanese niche hobby until 2004, when Wayne Gould, a retired Hong Kong judge from New Zealand, developed a computer program to generate Sudoku puzzles. He convinced The Times of London to publish them, and the puzzle quickly spread to newspapers worldwide. By 2005, Sudoku had become a global craze, with books, competitions, and digital versions proliferating. The first World Sudoku Championship was held in Lucca, Italy in 2006.

Today, Sudoku is published in newspapers and puzzle books in over 80 countries, and countless digital versions exist for every platform. It remains one of the most enduring and beloved logic puzzles ever created.