PriorityQueue in Java
Java provides a PriorityQueue class that implements a min-heap by default. It is part of the Collections Framework and offers O(log n) for insertion and removal.
Basic usage:
PriorityQueue pq = new PriorityQueue<>(); // min-heap
pq.offer(5);
pq.offer(1);
pq.offer(3);
System.out.println(pq.peek()); // 1 (smallest)
System.out.println(pq.poll()); // 1
System.out.println(pq.poll()); // 3
Max-Heap using Comparator
To create a max-heap, pass a reverse comparator.
PriorityQueue maxHeap = new PriorityQueue<>((a,b) -> b - a);
// or
PriorityQueue maxHeap2 = new PriorityQueue<>(Collections.reverseOrder());
Custom objects:
Either implement
Comparable or provide a Comparator.class Task implements Comparable {
int priority;
public int compareTo(Task other) {
return this.priority - other.priority; // min-heap by priority
}
}
PriorityQueue pq = new PriorityQueue<>();
Two Minute Drill
- PriorityQueue in Java is a min-heap by default.
- Use offer() for insert, poll() for remove, peek() for top.
- To get max-heap, provide reverse comparator.
- Time complexity: O(log n) for insert and remove, O(1) for peek.
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