Data Structures & Algorithms Notes Page

What’s a Priority Queue?

Definiton:

• A priority queue is like a line (queue) where each item has a priority value
• Items with higher priority come out FIRST, not just the ones who came in first

Example:

• Think of it like hospital patients:
  • A patient with a heart attack (high priority) goes before someone with a cold (low priority)

Code Example:

• Java’s built-in priority queue uses a min-heap by default
  • This means the smallest element comes out first

import java.util.PriorityQueue;

public class PriorityQueueExample 
{
    public static void main(String[] args) 
    {
        PriorityQueue<Integer> pq = new PriorityQueue<>();

        pq.add(30);
        pq.add(10);
        pq.add(20);

        System.out.println(pq.poll());  // Output: 10 (smallest)
        System.out.println(pq.peek());  // Output: 20 (next smallest)
    }
}

Common Methods/Functions:

• add(value) or offer(value) → Adds a value
• poll() → Removes and returns the top priority element
• peek() → Returns (but doesn’t return) the top element