Changing Java PriorityQueue to a Max PQ [duplicate]

问题

The Priority Queue implementation in the Java standard library appears to be a min Priority Queue which I found somewhat confusing. In order to turn it into a max one I created a custom comparator object.

Comparator<Integer> cmp = new Comparator<Integer>()
{
    public int compare( Integer x, Integer y )
    {
        return y - x;
    }
};

I was wondering if there was a more elegant solution. Essentially I wan't a generic priority queue that could be used to implement Dijkstras etc. I didn't even realise there would be ones which operated in reverse :/

回答1:

Use Java's Collections.reverseOrder() comparator.

Java Reference

回答2:

Here is a code snippet using Collections.reverseOrder()-

    PriorityQueue<Integer> maxPQ = new PriorityQueue<Integer>(20,Collections.reverseOrder());

You also need to provide the initial capacity of the Priority Queue (20 here) along with the Comparator.

回答3:

Not sure what you mean by elegant but when I want a PQ implemented like a MaxHeap (used in Dijkstra's) I just use an inline comparator constructor.

PriorityQueue<Integer> PQ= new PriorityQueue<Integer>(20, new Comparator<Integer>(){
            public int compare(Integer o1, Integer o2){
                return o2 - o1;
            }
        });

It's simple enough for anytime I'm looking for something simple and only want to use the Comparator once.

回答4:

If you have an existing comparator you could create a generic inversing comparator.

public class InverseComparator<T> implements Comparator<T> {
    private final Comparator<T> delegate;

    public InverseComparator(Comparator<T> delegate) {
        this.delegate = delegate;
    }

    public int compare(T x, T y) {
        return delegate(y, x);
    }
}

来源：https://stackoverflow.com/questions/3705881/changing-java-priorityqueue-to-a-max-pq

标签

java

priority-queue

standard-library