main/src/georegression/geometry/UtilPolygons2D_I32.java
public static void flip( Polygon2D_I32 a ) {
		int N = a.size();
		int H = N/2;

		for (int i = 1; i <= H; i++) {
			int j = N-i;
			Point2D_I32 tmp = a.vertexes.data[i];
			a.vertexes.data[i] = a.vertexes.data[j];
			a.vertexes.data[j] = tmp;
		}
	}
public static void flip( Polygon2D_I32 a ) {
		int N = a.size();
		int H = N/2;

		for (int i = 1; i <= H; i++) {
			int j = N-i;
			Point2D_I32 tmp = a.vertexes.data[i];
			a.vertexes.data[i] = a.vertexes.data[j];
			a.vertexes.data[j] = tmp;
		}
	}
Flips the order of points inside the polygon.  The first index will remain the same will otherwise be reversed

@param a Polygon of order 3 or more.