🔢 Sudoku — Backtracking Solver

Press Play to watch backtracking solve it, or switch to Solve-It-Yourself.

Puzzle

Speed: 65%

Backtracking beats brute force

empty cells51

brute force9^51

cells visited51

placements0

Legal-move pruning kills illegal branches instantly.

Backtracking Solver

1solve(board):
2  if no empty cell: solved
3  pick empty cell with the FEWEST legal digits
4  for each legal digit v there:
5    board[cell] = v; if solve(): return true
6    board[cell] = 0   // backtrack
7  return false

Sudoku Solver Visualizer: Watch Backtracking Fill the Grid

This Sudoku simulator turns the familiar puzzle into a backtracking-search visualizer. Press Solve and watch the algorithm pick the most-constrained empty cell (the minimum-remaining-values heuristic), place a legal digit, recurse, and undo choices at dead ends — the same constraint-satisfaction backtracking behind N-Queens, on a grid everyone knows. A search-space card contrasts the astronomical brute-force count (9 to the power of the empty cells) with the few hundred cells the pruned search actually visits. Switch to Solve-It-Yourself and the puzzle becomes playable: click an empty cell, tap a digit, and any clash within the same row, column, or 3x3 box lights up red instantly, with a celebration when every unit holds 1-9. Three difficulties show how sparser clues force more backtracking.

What you can do in this simulation

Watch the backtracking solver place digits (green) and undo dead ends (red) cell by cell, at any speed
Solve-It-Yourself mode: click a cell and tap a digit, with live row/column/box conflict highlighting and a win check
A search-space card compares brute force (9^empties) with the few hundred cells the pruned search visits
Pick Easy, Medium, or Hard puzzles and see sparser clues trigger far more backtracking
A synced pseudocode panel highlights the pick-cell / place / backtrack steps of the recursion

Concepts covered

sudoku solver · backtracking · constraint satisfaction · minimum remaining values · search tree pruning · recursion · exact cover

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1solve(board):

2 if no empty cell: solved

3 pick empty cell with the FEWEST legal digits

4 for each legal digit v there:

5 board[cell] = v; if solve(): return true

6 board[cell] = 0 // backtrack

7 return false

Sudoku Solver Visualizer: Watch Backtracking Fill the Grid

What you can do in this simulation

Watch the backtracking solver place digits (green) and undo dead ends (red) cell by cell, at any speed

Solve-It-Yourself mode: click a cell and tap a digit, with live row/column/box conflict highlighting and a win check

A search-space card compares brute force (9^empties) with the few hundred cells the pruned search visits

Pick Easy, Medium, or Hard puzzles and see sparser clues trigger far more backtracking

A synced pseudocode panel highlights the pick-cell / place / backtrack steps of the recursion