using System;

​

public class Program 

{ 

    static int[,] sudokuPuzzle = new int[9, 9] 

    { 

        {7, 8, 0, 4, 0, 0, 1, 2, 0}, 

        {6, 0, 0, 0, 7, 5, 0, 0, 9}, 

        {0, 0, 0, 6, 0, 1, 0, 7, 8}, 

        {0, 0, 7, 0, 4, 0, 2, 6, 0}, 

        {0, 0, 1, 0, 5, 0, 9, 3, 0}, 

        {9, 0, 4, 0, 6, 0, 0, 0, 5}, 

        {0, 7, 0, 3, 0, 0, 0, 1, 2}, 

        {1, 2, 0, 0, 0, 7, 4, 0, 0}, 

        {0, 4, 9, 2, 0, 6, 0, 0, 7} 

    }; 

   public static void Main(string[] args) 

    { 

        if (SolveSudoku(sudokuPuzzle)) 

            PrintGrid(sudokuPuzzle); 

        else

            Console.WriteLine("No solution exists"); 

    } 

​

    static bool SolveSudoku(int[,] grid) 

    { 

        int row, col; 

        if (!FindUnassignedLocation(grid, out row, out col)) 

            return true; 

        for (int num = 1; num <= 9; num++) 

        { 

            if (IsSafe(grid, row, col, num)) 

            { 

                grid[row, col] = num; 

                if (SolveSudoku(grid)) 

                    return true; 

                grid[row, col] = 0; 

            } 

        } 

        return false; 

    } 

​

    static bool FindUnassignedLocation(int[,] grid, out int row, out int col) 

    { 

        row = 0; 

        col = 0; 

        for (int i = 0; i < 9; i++) 

        { 

            for (int j = 0; j < 9; j++) 

            { 

                if (grid[i, j] == 0) 

                { 

                    row = i; 

                    col = j; 

                    return true; 

                } 

            } 

        } 

        return false; 

    } 

​

    static bool UsedInRow(int[,] grid, int row, int num) 

    { 

        for (int col = 0; col < 9; col++) 

        { 

            if (grid[row, col] == num) 

                return true; 

        } 

        return false; 

    } 

​

    static bool UsedInCol(int[,] grid, int col, int num) 

    { 

        for (int row = 0; row < 9; row++) 

        { 

            if (grid[row, col] == num) 

                return true; 

        } 

        return false; 

    } 

​

    static bool UsedInBox(int[,] grid, int boxStartRow, 

                                 int boxStartCol, int num) 

    { 

        for (int row = 0; row < 3; row++) 

        { 

            for (int col = 0; col < 3; col++) 

            { 

                if (grid[row + boxStartRow, 

                         col + boxStartCol] == num) 

                    return true; 

            } 

        } 

        return false; 

    } 

​

    static bool IsSafe(int[,] grid, int row, 

                        int col, int num) 

    { 

        return !UsedInRow(grid, row, num) && 

               !UsedInCol(grid, col, num) && 

               !UsedInBox(grid, row - row % 3 , 

                              col - col % 3, num) && 

               grid[row, col] == 0; 

    } 

​

    static void PrintGrid(int[,] grid) 

    { 

        for (int row = 0; row < 9; row++) 

        { 

            for (int col = 0; col < 9; col++) 

                Console.Write(grid[row, col] + " "); 

            Console.WriteLine(); 

        } 

    } 

}