How to use the Sudoku Solver Visualizer
- 1. Set it upType or paste 9 rows of 9 digits, with 0 or . for blanks.
- 2. RunPress Solve and replay.
- 3. ReplayStep through; the cell being tried is highlighted, and given clues are shaded.
Worked example
The sample is a well-known puzzle with 30 clues. Because the solver always picks the empty cell with the fewest legal digits, it fills this puzzle in 51 placements with 0 backtracks: every chosen cell has exactly one possible digit at that moment. The first row of the solution is 5 3 4 6 7 8 9 1 2. Harder puzzles need guesses, and you'll see the solver undo them when it reaches a cell with no legal digit.
How the solver works
For each empty cell it counts the digits allowed by the row, column and 3×3 box rules, then fills the cell with the fewest options first. If a digit leads to a dead end later, it's removed and the next digit is tried. This minimum-remaining-values rule is the main reason simple backtracking solves typical puzzles in milliseconds. Before solving, the tool checks the clues don't already break a rule, and that there are at least 17, the minimum for a unique solution.
For interviews
Sudoku Solver is a classic backtracking problem, alongside N-Queens and Word Search. Be ready to explain the validity check with three sets or bitmasks for rows, columns and boxes, and why choosing the most constrained cell first prunes the search.
Frequently asked questions
What format should the puzzle be in?
81 digits, 0 or . for blanks. Spaces and line breaks are ignored.
What if the puzzle has several solutions?
The solver shows the first one it finds.
Why only the first 3,000 steps?
Very hard puzzles can take thousands of steps; replaying more isn't useful.
Is the Sudoku solver visualizer free?
Yes. You can use the Sudoku solver visualizer without signing up. The first time you run it, we ask once for your name and WhatsApp number; after that the Sudoku solver visualizer and every other Uncodemy tool open straight away.