🧩 Sudoku Solver

A zero-dependency, single-file Sudoku solver built with pure HTML, CSS & JavaScript.
Instant solve via backtracking algorithm · Keyboard navigation · Fully responsive · No build step.

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Demo

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Features

• Instant solver - backtracking algorithm solves any valid puzzle in milliseconds
• 3 built-in puzzles - Easy, Medium, and Hard difficulties to get started immediately
• Manual entry - fill in your own puzzle from a newspaper, app, or anywhere
• Conflict detection - validate the board and highlights conflicting cells in red
• Keyboard-first UX - arrow keys navigate cells, digits fill them, Backspace erases
• On-screen numpad - full touch support for mobile users
• Smart highlighting - selecting a cell highlights its row, column, and 3×3 box
• Animated solve - filled cells pop in with a staggered reveal animation
• No scroll design - entire UI fits within one viewport, no scrolling required
• Zero dependencies - single index.html file, no npm, no bundler, no framework

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How It Works

The solver uses a recursive backtracking algorithm:

1. Scan the grid left-to-right, top-to-bottom for the first empty cell
2. Try placing digits 1-9, checking row, column, and 3×3 box constraints
3. Recurse into the next empty cell
4. If no digit fits, backtrack and try the next candidate in the previous cell
5. Repeat until the board is fully filled or declared unsolvable

isValid(board, row, col, num)
  -> check row   : no duplicate in board[row][0..8]
  -> check col   : no duplicate in board[0..8][col]
  -> check box   : no duplicate in the 3×3 region

Time complexity: O(9^m) where m = number of empty cells - optimized by early pruning at each step.

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Getting Started

No install. No build. Just open it.

git clone https://github.com/manishkrmahato/sudoku.git
cd sudoku
open index.html   # or double-click in your file explorer

Or visit the Live Demo directly.

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Usage

Action	How
Select a cell	Click it
Enter a digit	Type 1-9 or use the on-screen numpad
Erase a cell	Backspace / Delete / 0 or the ✕ button
Navigate	Arrow keys
Load a puzzle	Pick a difficulty → click Load Puzzle
Solve	Click Solve
Check for errors	Click Validate
Start fresh	Click Clear

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Project Structure

sudoku/
└── index.html      # Everything - layout, styles, and logic in one file
└── demo.gif        # Demo recording for README
└── README.md

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Lighthouse Score

Metric	Score
Performance	100
Accessibility	100
Best Practices	100
SEO	100

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Author

Manish Kumar Mahato
B.Tech CSE · NIT Warangal

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_{Built with pure HTML, CSS & JavaScript · No frameworks · No dependencies}