How to convert an arraylist to a max heap in java using priority queue

Question

I know we can create a max heap using priority queue using Collections.reverseOrder(), but I need to pass the ArrayList in that place also. I tried to create a custom comparator just in case, but it doesnt seem to work. I would like to know the exact syntax of doing so.
Example/My knowledge:
1)creating an empty min heap -> PriorityQueue pqmin = new PriorityQueue();
2)Creating a min heap from an ArrayList arr -> PriorityQueue pqmin = new PriorityQueue(arr);
3)Creating an empty max heap -> PriorityQueue pqmax = new PriorityQueue(Collections.reverseOrder());

My question: How to create max heap from existing arrayList using priorityQueue in Java

score 1 · Answer 1 · answered May 19 '20 at 07:02

1

There's no such constructor in PriorityQueue that takes both a collection and a comparator.

But you can use addAll method:

PriorityQueue pqmax = new PriorityQueue(Collections.reverseOrder());
pqmax.addAll(arr);

answered May 19 '20 at 07:02

Lev Leontev

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okay thank you, but will the time complexity of doing addAll be O(n) or O(nlogn) – Uddhav Navneeth May 20 '20 at 14:31
It is going to O(nlogn), because `PriorityQueue` has `addAll` inherited from `AbstractQueue`, where it just iterates over the collection. – Lev Leontev May 20 '20 at 15:43

score 1 · Answer 2 · answered May 19 '20 at 07:09

1

Use the 3), then addAll. Or slightly better is to pass initial capacity

 PriorityQueue pqmax = new PriorityQueue(arr.size(), Collections.reverseOrder()); 
 pqmax.addAll(arr);

answered May 19 '20 at 07:09

kan

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How to convert an arraylist to a max heap in java using priority queue

2 Answers2