hdu1509（Windows Message Queue） 优先队列

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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：

根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。从题目就很容易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：首先根据题意，可以建一个Message类，这里面包括mesg的一些信息，主要包括，优先级和进队的序列号（这个序列号必须要，也就是第几个进队的，后面就知道了）。然后根据指令一步一步的操作，就可以得到答案。

注意：

1、PriorityQueue类与普通队列最主要的区别就是多了个比较器。一般情况下，都是自己通过实现Comparator接口写一个比较器，在new 优先队列时将这个比较器丢进去就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，并根据指定的比较器对元素进行排序。

2、虽然优先队列在放入元素时，会通过其中的比较器进行比较后，放到相应的位置。但是至于它内部是怎么比较的，怎么放的，我还没去深究，但是在这里我知道，虽然题目是说先通过优先级进行存放，然后通过进队顺序存放，听起来只要考虑

优先级排序就行了，主观上就会以为只要优先级相同时，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，其实不然，并不是这样的，这里必须要通过进队的序列号来比较后才能达到想要的目的。

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{
			//这里如果没有id的比较，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;
			}
		}
	}
}