Question 1

What algorithm does this Sudoku solver use?

Accepted Answer

A classic backtracking search: it finds the first empty cell, tries each digit 1-9 that does not already appear in that row, column, or 3x3 box, and recurses. If a branch reaches a dead end, it undoes the last placement and tries the next digit. This runs entirely in your browser and solves virtually any valid 9x9 puzzle, including very hard ones, in well under a second.

Question 2

Does my puzzle need to have a unique solution?

Accepted Answer

No. If a puzzle has multiple valid solutions (for example, a mostly empty grid), the solver returns the first complete, valid solution it finds rather than every possibility. A well-formed Sudoku puzzle intended for humans normally has exactly one solution, but the solver does not require that to run.

Question 3

Why does it say my puzzle "has conflicting digits"?

Accepted Answer

This means two of the numbers you entered already break Sudoku's rules before any solving happens — either the same digit appears twice in one row, one column, or one 3x3 box. Use the Check for Conflicts button to see exactly which cells are highlighted, fix the typo, and solve again.

Question 4

Why does it say my puzzle "has no valid solution"?

Accepted Answer

This means your given digits don't individually repeat in any row, column, or box, but no combination of digits can complete the grid without eventually breaking a rule somewhere. This usually points to a typo in one of the givens — double-check each entered digit against the original puzzle.

Question 5

Is my puzzle uploaded anywhere?

Accepted Answer

No. Every digit you type stays in your browser. The solving and validation logic runs as plain JavaScript on your device — nothing is ever sent to a server.

Question 6

Can I paste a puzzle instead of typing each digit?

Accepted Answer

Yes, if you know the classic Sudoku string notation. Type or paste a row into each cell as usual, or fill the grid by tabbing across cells left-to-right, top-to-bottom — the same order a copied 81-character puzzle string (using 0 or . for blanks) follows.

Sudoku Solver

How to Use

Frequently Asked Questions

What algorithm does this Sudoku solver use?

Does my puzzle need to have a unique solution?

Why does it say my puzzle "has conflicting digits"?

Why does it say my puzzle "has no valid solution"?

Is my puzzle uploaded anywhere?

Can I paste a puzzle instead of typing each digit?

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