Changing Java PriorityQueue to a Max PQ [duplicate]

Question

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• Change priorityQueue to max priorityqueue 14 answers

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 :/

Answer 1

Use Java's Collections.reverseOrder() comparator.

Java Reference

Answer 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.

Answer 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.

Answer 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);
    }
}