如何在jgrapht DirectedAcyclicGraph中查找根和叶集合

import org.jgrapht.DirectedAcyclicGraph;
import org.jgrapht.graph.DefaultEdge;

DirectedAcyclicGraph<String, DefaultEdge> dag = new DirectedAcyclicGraph<>(DefaultEdge.class);

dag.addVertex("A");
dag.addVertex("B");
dag.addVertex("C");
dag.addVertex("D");

dag.addEdge("A", "B");
dag.addEdge("A", "C");
dag.addEdge("B", "D");
dag.addEdge("C", "D");

Set<String> roots = new HashSet<>();
for (String vertex : dag.vertexSet()) {
    if (dag.inDegreeOf(vertex) == 0) {
        roots.add(vertex);
    }
}

Set<String> leaves = new HashSet<>();
for (String vertex : dag.vertexSet()) {
    if (dag.outDegreeOf(vertex) == 0) {
        leaves.add(vertex);
    }
}

System.out.println("Roots: " + roots);
System.out.println("Leaves: " + leaves);