SudokuSolver.java

package main.java.com.thealgorithms.backtracking;

public class SudokuSolver {

    public static boolean sudokuSolver(int[][] sudoku, int row, int col) {
        if (row == 9) {
            return true; // base case: reached the end
        }

        int nextRow = row;
        int nextCol = col + 1;
        if (nextCol == 9) {
            nextRow = row + 1;
            nextCol = 0;
        }

        if (sudoku[row][col] != 0) {
            return sudokuSolver(sudoku, nextRow, nextCol);
        }

        for (int num = 1; num <= 9; num++) {
            if (isSafe(sudoku, row, col, num)) {
                sudoku[row][col] = num;
                if (sudokuSolver(sudoku, nextRow, nextCol)) {
                    return true;
                }
                sudoku[row][col] = 0; // backtrack
            }
        }
        return false;
    }

    private static boolean isSafe(int[][] sudoku, int row, int col, int num) {
        // Check row and column
        for (int i = 0; i < 9; i++) {
            if (sudoku[row][i] == num || sudoku[i][col] == num) {
                return false;
            }
        }

        // Check 3x3 subgrid
        int startRow = row - row % 3;
        int startCol = col - col % 3;
        for (int i = startRow; i < startRow + 3; i++) {
            for (int j = startCol; j < startCol + 3; j++) {
                if (sudoku[i][j] == num) {
                    return false;
                }
            }
        }

        return true;
    }

    private static void printSudoku(int[][] sudoku) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                System.out.print(sudoku[i][j] + " ");
            }
            System.out.println();
        }
    }

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

        if (sudokuSolver(sudoku, 0, 0)) {
            System.out.println("Sudoku Solved:");
            printSudoku(sudoku);
        } else {
            System.out.println("No solution exists.");
        }
    }
}