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 something like:

PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> -1 * Integer.compare(x, y));

Which works for any other base comparison function you might have.

Answer 2

Using lamda, just multiple the result with -1 to get max priority queue.

PriorityQueue<> q = new PriorityQueue<Integer>(
                       (a,b) ->  -1 * Integer.compare(a, b)
                    );

Answer 3

You can use lambda expression since Java 8.

The following code will print 10, the larger.

// There is overflow problem when using simple lambda as comparator, as pointed out by Фима Гирин.
// PriorityQueue<Integer> pq = new PriorityQueue<>((x, y) -> y - x);

PriorityQueue<Integer> pq =new PriorityQueue<>((x, y) -> Integer.compare(y, x));

pq.add(10);
pq.add(5);
System.out.println(pq.peek());

The lambda function will take two Integers as input parameters, subtract them from each other, and return the arithmetic result. The lambda function implements the Functional Interface, Comparator<T>. (This is used in place, as opposed to an anonymous class or a discrete implementation.)

Answer 4

You can use MinMaxPriorityQueue (it's a part of the Guava library): here's the documentation. Instead of poll(), you need to call the pollLast() method.

Answer 5

How about like this:

PriorityQueue<Integer> queue = new PriorityQueue<>(10, Collections.reverseOrder());
queue.offer(1);
queue.offer(2);
queue.offer(3);
//...

Integer val = null;
while( (val = queue.poll()) != null) {
    System.out.println(val);
}

The Collections.reverseOrder() provides a Comparator that would sort the elements in the PriorityQueue in a the oposite order to their natural order in this case.

Answer 6

Change PriorityQueue to MAX PriorityQueue Method 1 : Queue pq = new PriorityQueue<>(Collections.reverseOrder()); Method 2 : Queue pq1 = new PriorityQueue<>((a, b) -> b - a); Let's look at few Examples:

public class Example1 {
    public static void main(String[] args) {

        List<Integer> ints = Arrays.asList(222, 555, 666, 333, 111, 888, 777, 444);
        Queue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
        pq.addAll(ints);
        System.out.println("Priority Queue => " + pq);
        System.out.println("Max element in the list => " + pq.peek());
        System.out.println("......................");
        // another way
        Queue<Integer> pq1 = new PriorityQueue<>((a, b) -> b - a);
        pq1.addAll(ints);
        System.out.println("Priority Queue => " + pq1);
        System.out.println("Max element in the list => " + pq1.peek());
        /* OUTPUT
          Priority Queue => [888, 444, 777, 333, 111, 555, 666, 222]
          Max element in the list => 888
          ......................
           Priority Queue => [888, 444, 777, 333, 111, 555, 666, 222]
           Max element in the list => 888

         */


    }
}

Let's take a famous interview Problem : Kth Largest Element in an Array using PriorityQueue

public class KthLargestElement_1{
    public static void main(String[] args) {

        List<Integer> ints = Arrays.asList(222, 555, 666, 333, 111, 888, 777, 444);
        int k = 3;
        Queue<Integer> pq = new PriorityQueue<>(Collections.reverseOrder());
        pq.addAll(ints);
        System.out.println("Priority Queue => " + pq);
        System.out.println("Max element in the list => " + pq.peek());
        while (--k > 0) {
            pq.poll();
        } // while
        System.out.println("Third largest => " + pq.peek());
/*
 Priority Queue => [888, 444, 777, 333, 111, 555, 666, 222]
Max element in the list => 888
Third largest => 666

 */
    }
}

Another way :

public class KthLargestElement_2 {
    public static void main(String[] args) {
        List<Integer> ints = Arrays.asList(222, 555, 666, 333, 111, 888, 777, 444);
        int k = 3;

        Queue<Integer> pq1 = new PriorityQueue<>((a, b) -> b - a);
        pq1.addAll(ints);
        System.out.println("Priority Queue => " + pq1);
        System.out.println("Max element in the list => " + pq1.peek());
        while (--k > 0) {
            pq1.poll();
        } // while
        System.out.println("Third largest => " + pq1.peek());
        /*
          Priority Queue => [888, 444, 777, 333, 111, 555, 666, 222] 
          Max element in the list => 888 
          Third largest => 666

         */
    }
}

As we can see, both are giving the same result.