Java program is not showing output even though it compiles with no error

Question

I'm new to Java and I'm trying to build a sudoku solver programm using backtracking and an int[][] matrix.

Function usage:

1. isSafeSudoku()

   • this function checks whether the digit (from 1 to 9) is safe to place in correspondance to its row and its column and also its 3x3 grid.

2. sudokuSolver()

   • this function is recursive and it's the one which calls all other function

3. sudokuPrint()

   • this function simply prints the sudoku

public class sudoku {
// Function of sudoku solver ---------------------------------------
  public static boolean isSafeSudoku(int sudoku[][], int row, int col, int digit) {
    // is safe for that column ? vertically ?
    for (int i = 0; i < 9; i++) {
      if (sudoku[i][col] == digit) {
        return false;
      }
    }

    // is safe for that row ? horizontally ?
    for (int j = 0; j < 9; j++) {
      if (sudoku[row][j] == digit) {
        return false;
      }
    }

    // is safe for that grid ? that particular 3x3 grid
    int sr = (row / 3) * 3; // sr: starting row
    int sc = (col / 3) * 3; // sc: starting column
    // 3x3 grid
    for (int i = sr; i < sr + 3; i++) {
      for (int j = sc; j < sc + 3; j++) {
        if (sudoku[row][col] == digit) {
          return false;
        }
      }
    }

    return true;

  }

**// this is recursive function**
  public static boolean sudokuSolver(int sudoku[][], int row, int col) {
    // Base case
    if (row == 9) {
      return true;
    }

    // Recursion Part

    // A way to traverse in sudoku from col to col then next row and then again col
    // to col
    int nextRow = row, nextCol = col + 1;
    if (col + 1 == 9) {
      nextRow = row + 1;
      nextCol = 0;
    }

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

    // checker, if digit can be placed in the (row, col) or not?
    for (int digit = 1; digit <= 9; digit++) {
      if (isSafeSudoku(sudoku, row, col, digit)) {
        sudoku[row][col] = digit;
        if (sudokuSolver(sudoku, nextRow, nextCol)) { // if solution exist for digit for next row/col then do
          return true;
        }
        sudoku[row][col] = 0;
      }
    }

    return false;

  }

  public static void sudokuPrint(int sudoku[][]) {
    for (int row = 0; row < 9; row++) {
      for (int col = 0; col < 9; col++) {
        System.out.print(sudoku[row][col] + " ");
      }
      System.out.println();
    }
  }

  // Function of sudoku solver ends---------------------------------------

public static void main(String args[]) {

int sudoku[][] = {
        { 0, 0, 8, 0, 0, 0, 0, 0, 0 },
        { 4, 9, 0, 1, 5, 7, 0, 0, 2 },
        { 0, 0, 3, 0, 0, 4, 1, 9, 0 },
        { 1, 8, 5, 0, 6, 0, 0, 2, 0 },
        { 0, 0, 0, 0, 2, 0, 0, 6, 0 },
        { 9, 6, 0, 4, 0, 5, 3, 0, 0 },
        { 0, 3, 0, 0, 7, 2, 0, 0, 4 },
        { 0, 4, 9, 0, 3, 0, 0, 5, 7 },
        { 8, 2, 7, 0, 0, 9, 0, 1, 3 }
    };

    if (sudokuSolver(sudoku, 0, 0)) {
      System.out.println("Solution exist.");
    } else {
      System.out.println("Solution does not exist.");
    }

}

}

Answer 1

You should try -

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

the return keyword is missing in your solution. Without which your code flow is going to the next for loop to assign value to sudoku[row][col], even when there is a value ie. not equals to 0.