/usr/include/geos/triangulate/ is in libgeos++-dev 3.6.2-1build2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/geos/triangulate/DelaunayTriangulationBuilder.h
/usr/include/geos/triangulate/IncrementalDelaunayTriangulator.h
/usr/include/geos/triangulate/VoronoiDiagramBuilder.h
/usr/include/geos/triangulate/quadedge/
/usr/include/geos/triangulate/quadedge/LastFoundQuadEdgeLocator.h
/usr/include/geos/triangulate/quadedge/LocateFailureException.h
/usr/include/geos/triangulate/quadedge/QuadEdge.h
/usr/include/geos/triangulate/quadedge/QuadEdgeLocator.h
/usr/include/geos/triangulate/quadedge/QuadEdgeSubdivision.h
/usr/include/geos/triangulate/quadedge/TrianglePredicate.h
/usr/include/geos/triangulate/quadedge/TriangleVisitor.h
/usr/include/geos/triangulate/quadedge/Vertex.h