Sudoku

In Sudoku, every digit must appear once in each row, column, and box under the standard rules. Enter candidates, find naked and hidden singles, then use pairs, triples, locked candidates, and other eliminations as needed. Each placement should have a reason that remains valid across all three units. If a puzzle appears invalid, provide the full grid, given clues, variant rules, and the first contradiction or duplicate.

Before guessing, refresh your candidate marks after the most recent placement. Check each row, column, and box for a cell with one candidate, then for a digit that has only one possible position in that unit. These are different patterns; a cell may show several candidates while still being the only home for one digit.

If those scans find nothing, look for pairs that restrict other cells, and switch from box-by-box scanning to rows or columns. Recheck earlier notes for omissions or contradictions. Some difficult puzzles require more advanced deductions; the fact that basic techniques have stalled does not prove the puzzle is invalid. If available, use a hint to learn the next logical step rather than entering a random digit.

Try it at /games/sudoku.

A valid Sudoku solution fills each row, column, and 3×3 box with digits 1–9 exactly once. Begin by scanning for missing digits, then pencil in candidates. A naked single has one candidate in its cell; a hidden single is the only place for a digit in a row, column, or box. Pairs and other elimination patterns can carry you further.

Not every puzzle is easy to solve with a short list of human techniques, and some published puzzles are ambiguous or inconsistent. If stuck, recheck candidate notes and givens before guessing. For a computer-assisted solve, controlled backtracking can prove whether zero, one, or multiple solutions exist. In Sudoku, treat a solved board as valid only after checking all constraints.

Sudoku is a logic puzzle with one goal: each row, column, and 3×3 box must contain the digits 1–9 exactly once. A reliable solving routine reduces guesswork by keeping the candidate list current.

Scan rows, columns, and boxes for obvious placements. When that stalls, write the legal candidates in each empty cell. A naked single has only one candidate; a hidden single is the only place for a digit within a row, column, or box. After every placement, remove that digit from related candidates before searching again.

If two cells in one unit contain the same two candidates, those digits can be removed from the other cells in that unit. Check the pair really has exactly two candidates in both cells. If you are still stuck, revisit stale notes before guessing.

Practice a fresh grid at Sudoku.