SudokuSolver.java

public boolean solve(int row, int col) {

        if (row == 9) { // Solved!

            return true;
        }

        if (matrix[row][col] != 0) { // Cell not empty move to the other

            if (solve(col == 8 ? row + 1 : row, col == 8 ? 0 : col + 1)) {
                
                return true;
            }
            
        } 
        
        else {

            for (int i = 1; i < 10; i++) {

                if (isValid(row, col, i)) {

                    matrix[row][col] = i; // Try number

                    if (solve(col == 8 ? row + 1 : row, col == 8 ? 0 : col + 1)) {

                        return true;
                    } 
                    
                    else {

                        matrix[row][col] = 0; // Clean the cell and try again
                    }

                }

            }

        }

        return false;
    }