我正在尝试使用JGraphT实现Prim的最小生成树算法。看起来如何?
我遇到的一个问题是JGraphT对所指示内容的处理。因此,有时有必要进行一些笨拙的调用来反转g.getEdgeSource(e)
和g.getEdgeTarget(e)
(如果它们恰巧不正确)。
我最初尝试使用JGraphT的Fibonacci堆来实现这一点,但是它太难了,所以我只做了常规的PQ。
没有将不存在的边的权重设置为无穷大,我只是没有将其添加到队列中。
忠告?风格问题?效率低下吗?我应该使用代码而不是自己编写代码?
public static Graph<String, DefaultWeightedEdge> primPQ(final WeightedGraph<String, DefaultWeightedEdge> g, String root) {
Graph<String, DefaultWeightedEdge> mst = new SimpleWeightedGraph<String, DefaultWeightedEdge>(DefaultWeightedEdge.class);
Queue<DefaultWeightedEdge> pq = new PriorityQueue<DefaultWeightedEdge>(g.vertexSet().size(), new Comparator<DefaultWeightedEdge>() {
@Override
public int compare(DefaultWeightedEdge o1, DefaultWeightedEdge o2) {
if (g.getEdgeWeight(o1) < g.getEdgeWeight(o2)) {
return -1;
}
if (g.getEdgeWeight(o1) > g.getEdgeWeight(o2)) {
return 1;
}
return 0;
}
});
mst.addVertex(root);
DefaultWeightedEdge link;
for (String v : g.vertexSet()) {
link = g.getEdge(root, v);
if (link != null) {
pq.add(link);
}
}
//this is made difficult by JGraphT's assumption that everything is directed
DefaultWeightedEdge minEdge = pq.poll();
String toAdd;
String alreadyFound;
String tmp;
while (minEdge != null) {
// guessing at which is in the MST
toAdd = g.getEdgeTarget(minEdge);
alreadyFound = g.getEdgeSource(minEdge);
if (!(mst.containsVertex(toAdd) && mst.containsVertex(alreadyFound))) {
// swap if backwards
if (mst.containsVertex(toAdd)) {
tmp = toAdd;
toAdd = alreadyFound;
alreadyFound = tmp;
}
mst.addVertex(toAdd);
mst.addEdge(alreadyFound, toAdd, minEdge);
System.out.format("%s --> %s\n", g.getEdgeSource(minEdge), toAdd);
for (String v : g.vertexSet()) {
if (! mst.containsVertex(v)) {
link = g.getEdge(toAdd, v);
if (pq.contains(link)) {
g.setEdgeWeight(minEdge, Math.min(g.getEdgeWeight(minEdge), g.getEdgeWeight(link)));
}
if (link != null && ! pq.contains(link)) {
pq.add(link);
}
}
}
}
minEdge = pq.poll();
}
return mst;
}
最佳答案
我已经将您的算法的结果与一个作业进行比较,这两个作业都具有相同的最小总重量,请保持一致!