hdu1509（Windows Message Queue） 优先队列

Posted 2020-09-16 yxysuanfa

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Problem Description

Message queue is the basic fundamental of windows system. For each process, the system maintains a message queue. If something happens to this process, such as mouse click, text change, the system will add a message to the queue. Meanwhile, the process will do a loop for getting message from the queue according to the priority value if it is not empty. Note that the less priority value means the higher priority. In this problem, you are asked to simulate the message queue for putting messages to and getting message from the message queue.

 

Input

There‘s only one test case in the input. Each line is a command, "GET" or "PUT", which means getting message or putting message. If the command is "PUT", there‘re one string means the message name and two integer means the parameter and priority followed by. There will be at most 60000 command. Note that one message can appear twice or more and if two messages have the same priority, the one comes first will be processed first.(i.e., FIFO for the same priority.) Process to the end-of-file.

 

Output

For each "GET" command, output the command getting from the message queue with the name and parameter in one line. If there‘s no message in the queue, output "EMPTY QUEUE!". There‘s no output for "PUT" command.

 

Sample Input

GET
PUT msg1 10 5
PUT msg2 10 4
GET
GET
GET

 

Sample Output

EMPTY QUEUE!
msg2 10
msg1 10
EMPTY QUEUE!

题意：

依据操作指令进行操作，也就是出队，还是入队。而在出队时。要注意。保证优先级低的先出来，要是优先级一样的话。就先入队的先出来。从题目就非常easy看出要用优先队列做，恰好java包中有个PriorityQueue类，就是现成的优先队列。

解题过程：首先依据题意，能够建一个Message类，这里面包含mesg的一些信息，主要包含，优先级和进队的序列号（这个序列号必需要，也就是第几个进队的，后面就知道了）。然后依据指令一步一步的操作，就能够得到答案。

注意：

1、PriorityQueue类与普通队列最基本的差别就是多了个比較器。普通情况下，都是自己通过实现Comparator接口写一个比較器，在new 优先队列时将这个比較器丢进去就ok了，

构造方法中就有 PriorityQueue(int initialCapacity, Comparator<?

super E> comparator) 

           使用指定的初始容量创建一个 PriorityQueue，并依据指定的比較器对元素进行排序。

2、尽管优先队列在放入元素时，会通过当中的比較器进行比較后，放到对应的位置。可是至于它内部是怎么比較的，怎么放的。我还没去深究，可是在这里我知道。尽管题目是说先通过优先级进行存放，然后通过进队顺序存放，听起来仅仅要考虑

优先级排序即可了。主观上就会以为仅仅要优先级同样时，也就是compare返回0。它就会放在同一优先级。但先进来的元素后面。事实上不然，并非这种，这里必需要通过进队的序列号来比較后才干达到想要的目的。

代码实现：

import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Scanner;

public class P1509 {

	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		PriorityQueue<Message> priorityQue=new PriorityQueue<Message>(6000,new MesCmp());//优先队列
		Message messg;//messg类
		int count=0;
		while(sc.hasNext()){
			String operate=sc.next();
			if(operate.charAt(0)==‘G‘){//出队
				if(priorityQue.isEmpty()){
					System.out.println("EMPTY QUEUE!");
				}else{
					System.out.println(priorityQue.poll());
				}
			}else{//入队
				messg=new Message(sc.next(), sc.nextInt(), sc.nextInt(),count++);
				priorityQue.add(messg);
			}
		}
	}

}

class Message{
	String name;
	int value;
	int priority;
	int id;
	public Message(String name, int value, int priority,int id) {
		this.name = name;
		this.value = value;
		this.priority = priority;
		this.id = id;
	}
	public String toString() {
		return name + " " + value;
	}
}
class MesCmp implements Comparator<Message>{

	public int compare(Message m1, Message m2) {
		if(m1.priority<m2.priority){
			return -1;
		}else if(m1.priority>m2.priority){
			return 1;
		}else{
			//这里假设没有id的比較，那顺序就会出错
			if(m1.id<m2.id){
				return -1;
			}else if(m1.id>m2.id){
				return 1;
			}else{
				return 0;
			}
		}
	}
}