问题

I am working with Priority Queue problem in this link Hackkerank Problem

My Student class was created follow by OOP patterns which have constructor with 3 attributes (id, Name, cgpa) and some Getter method

My Priorities

class Priorities{
        private List<Student> list_student = new ArrayList<>();

    class  CheckCondition implements Comparator<Student>{


        @Override
        public int compare(Student o1, Student o2) {
            if(o1.getCGPA() < o2.getCGPA()){
                return 1;
            }else if(o1.getCGPA() > o2.getCGPA()){
                return -1;
            }else{
                if(o1.getName().compareTo(o2.getName()) < 0) {
                    return 1;
                }else if(o1.getName().compareTo(o2.getName()) > 0){
                    return -1;
                }else {
                    if(o1.getID() < o2.getID()){
                        return -1;
                    }else {
                        return 1;
                    }
                }
            }

        }
    }


    public List<Student> getStudents(List<String> events, int capacity){
        Queue<Student> stringQueue = new PriorityQueue<>(capacity, new CheckCondition());
        if(events.isEmpty()){
            System.out.println("EMPTY");
        }
        for(String element: events){
            List<String> stringList = List.of(element.split(" "));
            if(stringList.get(0).equalsIgnoreCase("ENTER")){
                Student new_student = new   Student(Integer.parseInt(stringList.get(3)), stringList.get(1), Double.parseDouble(stringList.get(2)));
                stringQueue.add(new_student);
            }if(stringList.get(0).equalsIgnoreCase("SERVED") ){
                stringQueue.poll();
            }
        }
        stringQueue.stream().forEach(element -> list_student.add(element));

        return list_student;

    }
}

My Main

public class Main {
public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         List<String> events = new ArrayList<>();
         Integer capacity = sc.nextInt();
         sc.nextLine();

        for (int i = 0; i < capacity; i++) {
            events.add(sc.nextLine());
        }
    
        Priorities priorities = new Priorities();
        List<Student> students = priorities.getStudents(events, capacity);
    
        for(Student element: students){
            System.out.println(element.getName());
        }
    
    }

}

So When I try with the test case in Hackerrank, My result instead of as expect, I change slightly in the end of the Queue

Expect result:

Dan
Ashley
**Shafaet
Maria**

Actual result:

Dan
Ashley
**Maria
Shafaet**

Can anyone help me to figure out why I can have this result as I try many times but it showed an abnormal case when program add the "Anik" student in the Queue which also change to position or 2 student "Shafaet" and "Maria".

Thank everyone so much

class Priorities{
private List<Student> list_student = new ArrayList<>();
class  CheckCondition implements Comparator<Student>{
@Override
public int compare(Student o1, Student o2) {
if(o1.getCGPA() < o2.getCGPA()){
return 1;
}else if(o1.getCGPA() > o2.getCGPA()){
return -1;
}else{
if(o1.getName().compareTo(o2.getName()) < 0) {
return 1;
}else if(o1.getName().compareTo(o2.getName()) > 0){
return -1;
}else {
if(o1.getID() < o2.getID()){
return -1;
}else {
return 1;
}
}
}
}
}
public List<Student> getStudents(List<String> events, int capacity){
Queue<Student> stringQueue = new PriorityQueue<>(capacity, new CheckCondition());
if(events.isEmpty()){
System.out.println("EMPTY");
}
for(String element: events){
List<String> stringList = List.of(element.split(" "));
if(stringList.get(0).equalsIgnoreCase("ENTER")){
Student new_student = new   Student(Integer.parseInt(stringList.get(3)), stringList.get(1), Double.parseDouble(stringList.get(2)));
stringQueue.add(new_student);
}if(stringList.get(0).equalsIgnoreCase("SERVED") ){
stringQueue.poll();
}
}
stringQueue.stream().forEach(element -> list_student.add(element));
return list_student;
}
}

public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
List<String> events = new ArrayList<>();
Integer capacity = sc.nextInt();
sc.nextLine();
for (int i = 0; i < capacity; i++) {
events.add(sc.nextLine());
}
Priorities priorities = new Priorities();
List<Student> students = priorities.getStudents(events, capacity);
for(Student element: students){
System.out.println(element.getName());
}
}
}

Dan
Ashley
**Shafaet
Maria**

Dan
Ashley
**Maria
Shafaet**

答案1

得分: 2

名称应按升序排序，因此当 o1 的名称在词典上位于 o2 之前时，应返回 -1。

if (o1.getName().compareTo(o2.getName()) < 0) {
    return -1;
} else if (o1.getName().compareTo(o2.getName()) > 0) {
    return 1;
}

此外，当id相等时，应返回 0。

另一个主要问题是 PriorityQueue 的 stream 方法不保证顺序。您可以轮询所有元素并添加到 List 中。

while (!stringQueue.isEmpty())
    list_student.add(stringQueue.poll());

请注意，使用 Comparator.comparing 方法可以更简单地编写 Comparator：

Queue<Student> stringQueue = new PriorityQueue<>(capacity, 
        Comparator.comparingDouble(Student::getCGPA).reversed()
            .thenComparing(Student::getName)
            .thenComparing(Student::getID));

if (o1.getName().compareTo(o2.getName()) < 0) {
	return -1;
} else if (o1.getName().compareTo(o2.getName()) > 0) {
	return 1;
}

while (!stringQueue.isEmpty())
	list_student.add(stringQueue.poll());

Queue<Student> stringQueue = new PriorityQueue<>(capacity, 
		Comparator.comparingDouble(Student::getCGPA).reversed()
            .thenComparing(Student::getName)
			.thenComparing(Student::getID));