我使用jgrapht在内存图中保存大约150000个顶点。这是一个有向图，每个顶点都有{0 | 1}个出边。
我想找到一组没有外边的顶点。以下是我的尝试：

import io.vavr.Tuple;
import io.vavr.Tuple2;
import io.vavr.Tuple3;
import io.vavr.control.Option;
import org.jgrapht.Graphs;
import org.jgrapht.graph.SimpleDirectedGraph;
import org.jgrapht.graph.concurrent.AsSynchronizedGraph;

 public Option<io.vavr.collection.Set<Employee>> 
 pickEmployeesWithNoSupervisor(Integer companyID) {

       // holdingCompany is of type: SimpleDirectedGraph<Employee,String>
       // edge is a simple string: "reportingTo"
       // Retrieve from a Map, initialized earlier, elsewhere

       var holdingCompany = this.allCompanies.get(companyID);
       if (holdingCompany == null)
                    return (Option.none());
       else {

           var vertices = holdingCompany.vertexSet();
           io.vavr.collection.Set<Employee> accumulator = io.vavr.collection.HashSet.empty();
           var allNoReportingToEmployees = 
                io.vavr.collection.HashSet.ofAll(vertices)
                .foldLeft(accumulator,(accu,nextEmp) -> {
                   var hasPredecessors = 
                          Graphs.vertexHasPredecessors(mayBeAKnownCompany,nextEmp);
                   return (!hasPredecessors ? accu.add(nextEmp) : accu) ;
                });

            return Option.some(allNoReportingToEmployees);
          }
     }

     public class Employee {
        private final Integer empID;

        public Employee(Integer empID) {
            this.empID = empID;
        }

        @Override
        public boolean equals(Object o) {
           // ..
        }

        @Override
        public int hashCode() {
            // ..
        }
    }

这也许是一种天真的尝试。我很想知道有没有更好、更地道、更有效的方法来做这件事。