Priority Queues

In the previous lessons, we learned about the basic queue data structure and its implementations. Now, let's move on to priority queues.

A priority queue is a variation of a queue where each element has a priority associated with it. The priority determines the order in which the elements are processed. The element with the highest priority is processed first.

Priority queues have the following characteristics:

• Elements are dequeued in order of their priority.
• When enqueuing elements, they are inserted based on their priority.
• Elements with the same priority are dequeued in the order they were enqueued.

Unlike a basic queue, which is a FIFO (First In First Out) data structure, a priority queue is an ordered data structure. Depending on the implementation, elements can be ordered in either ascending or descending order of their priorities.

Let's take a look at an example of a priority queue in Java:

1class Main {
2  public static void main(String[] args) {
3    // Priority Queue Example
4    PriorityQueue<Integer> pq = new PriorityQueue<>();
5    
6    pq.add(5);
7    pq.add(3);
8    pq.add(8);
9    pq.add(1);
10    pq.add(10);
11    
12    while (!pq.isEmpty()) {
13      System.out.println(pq.poll());
14    }
15  }
16}

Output:

11
23
35
48
510

In this example, we create a priority queue and add a few elements to it. The priority queue automatically orders the elements based on their values. When we dequeue the elements using the poll() method, they are returned in ascending order.

Priority queues are commonly used in various applications. For example:

• Task Scheduling: Priority queues can be used to schedule tasks in an application based on their priority levels.
• Event-driven Systems: Priority queues can be used to handle events based on their priority, ensuring that high-priority events are processed first.
• Graph Algorithms: Priority queues are often used in graph algorithms like Dijkstra's algorithm and Prim's algorithm to select the next vertex or edge to visit based on their priority.

By using a priority queue, you can efficiently process elements based on their priorities and optimize various algorithms and systems.

Now that we have a basic understanding of priority queues, let's explore their implementation and operations in more detail.

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class Main {

  public static void main(String[] args) {

    // Priority Queue Example

    PriorityQueue<Integer> pq = new PriorityQueue<>();

    pq.add(5);

    pq.add(3);

    pq.add(8);

    pq.add(1);

    pq.add(10);

    while (!pq.isEmpty()) {

      System.out.println(pq.poll());

    }

  }

}