Fill the 9×9 grid so every row, column, and 3×3 box contains the digits 1–9 exactly once. That's it. No adding, no multiplying — just logic.
Look at each number (1 through 9) and find where it can possibly go. For example: if 7 already appears in 2 rows of a box, it can only go in the third row of that box. Scan each number across all rows, columns, and boxes before trying anything complex.
A "naked single" is a cell where only one number can possibly fit. After eliminating all numbers already in that cell's row, column, and box — if only one possibility remains, fill it in immediately. Always scan for naked singles first — they're the fastest wins.
A "hidden single" is when a number can only go in one specific cell within a row, column, or box — even if that cell has other candidates. Look at each box: if the number 4 can only fit in one cell of that box, place it there — even if that cell also shows 2, 7, or 9 as options.
For cells you can't solve immediately, write small candidate numbers in the corners. As you fill in other cells, eliminate candidates from related cells. This turns complex puzzles into a simple process of elimination.
If all candidates for a number in a box are in the same row or column, that number can't appear elsewhere in that row/column — even in other boxes. This lets you eliminate candidates from cells outside the box. This one technique unlocks most medium-difficulty puzzles.
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