SudokuBacktracking.groovy

    private void nextCell() {
        if (columnIndex < this.board.size() - 1) {
            columnIndex++
        } else {
            columnIndex = 0
            rowIndex++
        }
    }

    private void previousCell() {
        if (columnIndex > 0) {
            columnIndex--
        } else {
            columnIndex = this.board.size() - 1
            rowIndex--
        }
    }