Learning-Java/SudokuSolver.java at main · expp121/Learning-Java · GitHub

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class SudokuSolver {

    public static boolean isSafe(int[][] board, int row, int col, int num){

        // row has the unique (row-clash)
        for (int d = 0; d < board.length; d++)
        {
            // if the number we are trying to
            // place is already present in
            // that row, return false;
            if (board[row][d] == num)
            {
                return false;
            }
        }

        // column has the unique numbers (column-clash)
        for (int r = 0; r < board.length; r++)
        {
            // if the number we are trying to
            // place is already present in
            // that column, return false;

            if (board[r][col] == num)
            {
                return false;
            }
        }

        // corresponding square has
        // unique number (box-clash)
        int sqrt = (int) Math.sqrt(board.length);
        int boxRowStart = row - row % sqrt;
        int boxColStart = col - col % sqrt;

        for (int r = boxRowStart; r < boxRowStart + sqrt; r++){

            for (int d = boxColStart; d < boxColStart + sqrt; d++){
                if (board[r][d] == num)
                    return false;
            }
        }
            // if there is no clash, it's safe
        return true;
    }

    public static boolean solveSudoku(int[][] board, int n){

        int row = -1;
        int col = -1;
        boolean isEmpty = true;
        for (int i = 0; i < n; i++){

            for (int j = 0; j < n; j++){

                if (board[i][j] == 0){
                    row = i;
                    col = j;

                    // we still have some remaining
                    // missing values in Sudoku
                    isEmpty = false;
                    break;
                }
            }

            if (!isEmpty)
                break;
        }

        // no empty space left
        if (isEmpty)
        {
            return true;
        }

        // else for each-row backtrack
        for (int num = 1; num <= n; num++)
        {
            if (isSafe(board, row, col, num))
            {
                board[row][col] = num;
                if (solveSudoku(board, n))
                {
                    // print(board, n);
                    return true;
                }
                else
                {
                    board[row][col] = 0; // replace it
                }
            }
        }
        return false;
    }

    public static void print(int[][] board, int N)
    {
        // we got the answer, just print it
        for (int r = 0; r < N; r++)
        {
            for (int d = 0; d < N; d++)
            {
                System.out.print(board[r][d]);
                System.out.print(" ");
            }
            System.out.print("\n");

            if ((r + 1) % (int) Math.sqrt(N) == 0)
            {
                System.out.print("");
            }
        }
    }

// Driver Code
    public static void main(String args[])
    {

        int[][] board = new int[][]
        {
                {3, 0, 6,   5, 0, 8,    4, 0, 0},
                {5, 2, 0,   0, 0, 0,    0, 0, 0},
                {0, 8, 7,   0, 0, 0,    0, 3, 1},

                {0, 0, 3,   0, 1, 0,    0, 8, 0},
                {9, 0, 0,   8, 6, 3,    0, 0, 5},
                {0, 5, 0,   0, 9, 0,    6, 0, 0},

                {1, 3, 0,   0, 0, 0,    2, 5, 0},
                {0, 0, 0,   0, 0, 0,    0, 7, 4},
                {0, 0, 5,   2, 0, 6,    3, 0, 0}
        };

        int N = board.length;

        if (solveSudoku(board, N))
        {
            print(board, N); // print solution
        }
        else
        {
            System.out.println("No solution");
        }
    }
}

// This code is contributed
// by MohanDas