for (int i = 1; i + 1 < polygon.size(); ++i) {
    const Point_2& v0 = polygon[0];
    const Point_2& v1 = polygon[i];
    const Point_2& v2 = polygon[i + 1];
}

Alpha_shape_2 aShape(inPoints.begin(), inPoints.end());
aShape.set_alpha( aShape.find_optimal_alpha(1) );
aShape.set_mode(Alpha_shape_2::REGULARIZED);