Sudoku Solver (C++)

A simple Sudoku solver written in C++17 using a classic backtracking algorithm.

The project is structured with a clear separation between:

• Sudoku: board representation, parsing, and printing
• Solver: solving logic and solvability checking

---

Features

• Solves standard 9×9 Sudoku puzzles
• Input as an 81-character string ('0' = empty cell)
• Pretty console output
• Input validation with exceptions
• Clean object-oriented design
• No global variables

---

Example

Input:

000030290
009000007
200008301
080500400
600000009
004007020
407100006
900000700
023040000

Output:

+-------+-------+-------+
| 7 8 4 | 1 3 6 | 2 9 5 |
| 3 5 9 | 2 4 8 | 1 6 7 |
| 2 6 1 | 9 5 7 | 8 4 3 |
+-------+-------+-------+
| 8 3 2 | 5 9 1 | 4 7 6 |
| 6 7 5 | 4 8 2 | 3 1 9 |
| 4 9 1 | 7 6 3 | 5 2 8 |
+-------+-------+-------+
| 5 4 7 | 3 1 9 | 6 8 2 |
| 9 2 8 | 6 7 4 | 5 3 1 |
| 1 6 3 | 8 2 5 | 9 7 4 |
+-------+-------+-------+

---

Build

Requirements

• C++17 compatible compiler (e.g. g++)

Compile

g++ main.cpp Sudoku.cpp Solver.cpp -std=c++17 -Wall -Wextra -o sudoku

---

Run

./sudoku

---

Usage

Edit main.cpp and provide a Sudoku puzzle as an 81-character string:

const std::string sudokuString =
    "000030290"
    "009000007"
    "200008301"
    "080500400"
    "600000009"
    "004007020"
    "407100006"
    "900000700"
    "023040000";

Rules:

• '0' represents an empty cell
• Digits '1'–'9' represent fixed values
• The string must contain exactly 81 characters

---

Design Overview

Sudoku

• Stores the board as std::array<int, 81>
• Parses input from an 81-character string (fromString)
• Provides accessors (at, setAt)
• Prints the board in a human-readable format
• Contains no solving logic

Solver

• Implements a recursive backtracking algorithm
• Computes empty positions once and reuses them during recursion
• Provides:
  • solve(Sudoku&) (modifies the board)
  • isSolvable(const Sudoku&) (checks solvability by solving a copy)

---

Error Handling

Invalid input throws std::invalid_argument:

• Sudoku string must be exactly 81 characters
• Only digits '0'–'9' are allowed

Example:

try {
    Sudoku s("invalid_input");
} catch (const std::invalid_argument& e) {
    std::cerr << e.what() << '\n';
}

---

License

MIT License. See 'LICENSE' for details.