SudokuSolver.java

import java.util.Scanner;

public class SudokuSolver {

    // Print the board
    public static void printBoard(int[][] board) {
        for (int i = 0; i < 9; i++) {
            if (i % 3 == 0 && i != 0) {
                System.out.println("------+-------+------");
            }
            for (int j = 0; j < 9; j++) {
                if (j % 3 == 0 && j != 0) {
                    System.out.print("| ");
                }
                System.out.print(board[i][j] == 0 ? ". " : board[i][j] + " ");
            }
            System.out.println();
        }
    }

    // Check if number is valid
    public static boolean isValid(int[][] board, int row, int col, int num) {
        // Check row & column
        for (int i = 0; i < 9; i++) {
            if (board[row][i] == num || board[i][col] == num)
                return false;
        }
        // Check 3x3 box
        int boxRow = row - row % 3;
        int boxCol = col - col % 3;
        for (int i = boxRow; i < boxRow + 3; i++) {
            for (int j = boxCol; j < boxCol + 3; j++) {
                if (board[i][j] == num)
                    return false;
            }
        }
        return true;
    }

    // Backtracking solver
    public static boolean solveSudoku(int[][] board) {
        for (int row = 0; row < 9; row++) {
            for (int col = 0; col < 9; col++) {
                if (board[row][col] == 0) { // empty cell
                    for (int num = 1; num <= 9; num++) {
                        if (isValid(board, row, col, num)) {
                            board[row][col] = num;
                            if (solveSudoku(board)) {
                                return true;
                            }
                            board[row][col] = 0; // backtrack
                        }
                    }
                    return false; // no valid number found
                }
            }
        }
        return true; // solved
    }

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int[][] board = new int[9][9];

        System.out.println("Enter Sudoku puzzle (0 for empty cells) row by row:");
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                board[i][j] = sc.nextInt();
            }
        }

        System.out.println("\nInput Puzzle:");
        printBoard(board);

        if (solveSudoku(board)) {
            System.out.println("\nSolved Sudoku:");
            printBoard(board);
        } else {
            System.out.println("\nNo solution exists for this Sudoku.");
        }

        sc.close();
    }
}