Change priorityQueue to max priorityqueue

Question

I have priority queue in Java of Integers:

 PriorityQueue pq= new PriorityQueue();

When I call pq.poll(

Answer 1

You can try pushing elements with reverse sign. Eg: To add a=2 & b=5 and then poll b=5.

PriorityQueue<Integer>  pq = new PriorityQueue<>();
pq.add(-a);
pq.add(-b);
System.out.print(-pq.poll());

Once you poll the head of the queue, reverse the sign for your usage. This will print 5 (larger element). Can be used in naive implementations. Definitely not a reliable fix. I don't recommend it.

Answer 2

PriorityQueue<Integer> lowers = new PriorityQueue<>((o1, o2) -> -1 * o1.compareTo(o2));

Answer 3

PriorityQueue<Integer> pq = new PriorityQueue<Integer> (
  new Comparator<Integer> () {
    public int compare(Integer a, Integer b) {
       return b - a;
    }
  }
);

Answer 4

This can be achieved by the below code in Java 8 which has introduced a constructor which only takes a comparator.

PriorityQueue<Integer> maxPriorityQ = new PriorityQueue<Integer>(Collections.reverseOrder());

Answer 5

Here is a sample Max-Heap in Java :

PriorityQueue<Integer> pq1= new PriorityQueue<Integer>(10, new Comparator<Integer>() {
public int compare(Integer x, Integer y) {
if (x < y) return 1;
if (x > y) return -1;
return 0;
}
});
pq1.add(5);
pq1.add(10);
pq1.add(-1);
System.out.println("Peek: "+pq1.peek());

The output will be 10

Answer 6

This can be achieved by using

PriorityQueue<Integer> pq = new PriorityQueue<Integer>(Collections.reverseOrder());