我正在研究有向无环图中的循环检测,并使用DFS实现了我的算法,以下是代码:
public class DFS {
public static boolean hasCycle(Graph graph) {
for (Vertex vertex : graph.getVertices()) {
if (detectCycle(graph, vertex)) {
return true;
}
}
return false;
}
private static boolean detectCycle(Graph graph, Vertex root) {
for (Vertex vertex : graph.getVertices()) {
vertex.setBeingVisited(false);
vertex.setVisited(false);
}
return DFS.dfs(graph, root);
}
private static boolean dfs(Graph graph, Vertex root) {
root.setBeingVisited(true);
for (Edge edge : root.getNeighbors()) {
Vertex neighborVertex = edge.getEndPoint();
if (neighborVertex.isBeingVisited()) {
return true;
} else if (!neighborVertex.isVisited()) {
DFS.dfs(graph, neighborVertex);
}
}
root.setVisited(true);
root.setBeingVisited(false);
return false;
}
}
以下是测试类:
public class App {
public static void main(String[] args) {
Vertex a = new Vertex("A");
Vertex b = new Vertex("B");
Vertex c = new Vertex("C");
Vertex d = new Vertex("D");
Vertex e = new Vertex("E");
Edge e1 = new Edge(a, b);
Edge e2 = new Edge(a, c);
a.addNeighbor(e1);
a.addNeighbor(e2);
Edge e3 = new Edge(b, d);
b.addNeighbor(e3);
Edge e4 = new Edge(c, e);
c.addNeighbor(e4);
Edge e5 = new Edge(d, a);
d.addNeighbor(e5);
List<Vertex> vertices = new ArrayList<>();
vertices.add(a);
vertices.add(b);
vertices.add(c);
vertices.add(d);
vertices.add(e);
Graph graph = new Graph(vertices);
System.out.println(DFS.hasCycle(graph));
if (DFS.hasCycle(graph)) {
System.out.println("It does have a cycle!");
} else {
System.out.println("It doesn't have cycle!");
}
}
}
针对这个特定案例,结果应该打印出图形具有循环的信息,因为我们要处理的图形如下:
>b --> d
/ |
a<-------
\
> c --> e
但是我的输出是错误的,就好像没有循环一样,我只是找不到我的实现中的错误。有人能帮忙吗?
root
的beingVisited
和visited
属性,这将导致其失败:一个简单的失败示例是a->b->c,并按顺序处理(b, a, c):它将说它有循环。请正确阅读我的答案:仅当DFS.dfs(graph, neighborVertex)
返回true
时才返回true
。如果为false
,则继续处理其他邻居。我希望现在清楚了。 - ram