I was trying to implement MST with Priorityqueue with custom comparator, but I am facing problem in building min-heap with it in O(n) time. The problem is only one constructor of Priorityqueue allows to create PriorityQueue in O(n), but it does not take any comparator as argument. I want it to use my custom comparator. Is there workaround for this problem ? PriorityQueue.addAll() will lose the purpose of using Min-heap for MST as it is O(nlogn) method. Here is my code.
ArrayList <edge>ar=new ArrayList<>();
for(int i=0;i<e;i++)
{
int u=ss.nextInt();
int v=ss.nextInt();
int w=ss.nextInt();
ar.add(new edge(u,v,w));
}
PriorityQueue <edge>pr=new PriorityQueue<edge>(ar);
And the comparator that I want to use:-
PriorityQueue <edge>ar=new PriorityQueue(11,new Comparator() {
@Override
public int compare(Object o1, Object o2) {
edge n1=(edge) o1;
edge n2=(edge) o2;
if(n1.w<n2.w)
{
return -1;
}
else if(n1.w==n2.w)
{
if((n1.u+n1.v+n1.w)<=(n2.u+n2.v+n2.w))
{
return -1;
}
else
{
return 1;
}
}
else
{
return 1;
}
}
});
If you have not min-heap-ordered your list elsewhere, you will not be able to new PriorityQueue(...) anything and somehow avoid the hit of creating your heap. The math here says it's O(n) for the average case, but it is still more than just iterating.
PriorityQueue<edge> pr = new PriorityQueue<edge>(ar, comp) {
PriorityQueue(List<edge> ar, Comparator<edge> c) {
this(c);
for(int i = 0; i < queue.length; i++) {
queue[i] = ar.get(i);
}
this.size = queue.length;
heapify(); // O(n), except that heapify is private and thus you can't call it!!!
}
}
Now I haven't tested this, it's just off the top of my head with some guidance from the PriorityQueue source, but it should point you in the right direction.
But sometime you have to pay the piper and create the heap-order and that be more than just iteration. It should still be on O(n) though, because of heapify.
Another option is to have edge implement Comparable<edge>. Then you can just PriorityQueue<edge> pr = new PriorityQueue(ar);
If you cannot control edge implements Comparable<edge> then you could compose a container class:
class EdgeContainer implements Comparable<EdgeContainer> {
private static final Comparator<edge> comp = ; // that comparator above
private final edge edge;
EdgeContainer(Edge edge) { this.edge = edge; }
public int compareTo(EdgeContainer e) { return comp.compare(edge, e.edge); }
public edge getEdge() { return edge; }
}
List <EdgeContainer>ar=new ArrayList<>();
for(int i=0;i<e;i++)
{
int u=ss.nextInt();
int v=ss.nextInt();
int w=ss.nextInt();
ar.add(new EdgeContainer(new edge(u,v,w)));
}
PriorityQueue<EdgeContainer> qr = new PriorityQueue(ar);