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.
When singles and pairs stop producing progress, check for locked candidates. If a digit inside one 3x3 box can appear only in a single row or column, eliminate that digit from the rest of the same line outside the box. That small elimination often exposes another single and starts a cascade. Re-scan the entire grid after every confirmed placement; difficult puzzles are usually solved by chaining modest deductions, not by one spectacular trick.
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