SudokuSolver.java

package Recursion;

public class SudokuSolver {

    static boolean isSafe(int[][] board, int row, int col, int num){
        for (int i = 0; i < board.length; i++) {
            if (board[i][col] == num) {
                return false;
            }
        }
        for (int i = 0; i < board[0].length; i++) {
            if (board[row][i] == num) {
                return false;
            }
        }
        int startRow = row - row % 3;
        int startCol = col - col % 3;
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                if (board[startRow + i][startCol + j] == num) {
                    return false;
                }
            }
        }
        return true;
    }

    static void display(int[][] board){
        System.out.println("-------------------------");
        for (int i = 0; i < board.length; i++) {
            System.out.print("| ");
            for (int j = 0; j < board[0].length; j++) {
                System.out.print(board[i][j]+" ");
                if (j % 3 == 2) {
                    System.out.print("| ");
                }
            }
            System.out.println();
            if (i % 3 == 2) {
                System.out.println("-------------------------");
            }
        }
        System.out.println();
    }

    static void sudoku(int[][] board, int row, int col){
        if (row >= board.length){
            display(board);
            return;
        }
        if (col >= board[0].length) {
            sudoku(board, row + 1, 0);
            return;
        }
        if (board[row][col] != 0) {
            sudoku(board, row, col + 1);
            return;
        }
        for (int i = 1; i < 10; i++) {
            if (isSafe(board, row, col, i)) {
                board[row][col] = i;
                sudoku(board, row, col + 1);
                board[row][col] = 0;
            }
        }
    }

    public static void main(String[] args) {
        int[][] sudokuBoard = {
                {7, 0, 0, 0, 0, 0, 0, 0, 1},
                {0, 0, 5, 9, 0, 0, 0, 0, 0},
                {0, 0, 0, 0, 0, 0, 0, 0, 0},
                {1, 0, 0, 0, 8, 3, 0, 0, 0},
                {0, 0, 0, 6, 0, 0, 9, 7, 0},
                {0, 2, 0, 0, 0, 0, 6, 0, 0},
                {3, 0, 0, 0, 1, 0, 0, 0, 0},
                {0, 0, 0, 4, 0, 0, 0, 9, 0},
                {0, 0, 0, 0, 0, 0, 5, 0, 0}
        };
        sudoku(sudokuBoard, 0, 0);
    }
}