选择排序双链表Java

Question

I need to Sort Linked list using Selection sort. But I can not use Collections. I have a troubles with finding smallest elements and creating a new version of sorted list. Thanks.

    public class LinkedList {
        public Node first;
        public Node last;

        public LinkedList() {
            first = null;
            last = null;
        }

        public boolean isEmpty() {
            return first == null;
        }

        public void addFirst(Student student) {
            Node newNode = new Node(student);
            if (isEmpty())
                last = newNode;
            else
                first.previous = newNode;
            newNode.next = first;
            first = newNode;
        }

        public void addLast(Student student) {
            Node newNode = new Node(student);
            if (isEmpty())
                first = newNode;
            else
                last.next = newNode;
            newNode.previous = last;
            last = newNode;
        }

        public void display() {
            Node current = last;
            while (current != null) {
                System.out.print(current.student.name + "\b");
                System.out.print(current.student.surname + "\b");
                System.out.println(current.student.educationType);
                current = current.previous;
            }
        }

Because of non-working findSmallest method Sort method doesn't work correctly. I try to implelent sorting by creating a new list where I put Nodes in sorted way. And it also doesn't go out of "While loop"

        public void Sort() {
            LinkedList list = new LinkedList();
            Node toStart = last;
            while (toStart!=null){
                list.addLast(findSmallest(toStart).student);
                toStart = toStart.previous;
            }


        }

It sends biggest element added and if I manually assign'last' to'smallest' it would work.

        public Node findSmallest(Node toStartFrom) {
            Node current = toStartFrom;
            Node smallest = toStartFrom; //if i put here `last` it will work correctly
            while(current != null) {
                if (smallest.student.name.compareToIgnoreCase(current.student.name) > 0) smallest = current;
                current = current.previous;
            }
            return smallest;
        }

    }

  public class Node {
        public Student student;

        public Node next;
        public Node previous;

        public Node(Student student) {
            this.student = student;
        }
    }


    public class Student {
        public String name;
        public String surname;
        public String educationType;

        static public Student createStudent() {
         ....
            return student;
        }
    }

Answer 1

It might be helpful to not have a doubly linked list because then you have less links that you need to maintain. Also you might be having trouble with the findSmallest() method because you are initially setting you current and smallest to the same node so when the if(smallest.student.name.compareToIgnoreCase(current.student.name) > 0) statement is executed you are comparing the name of the student to the same name of the student. For example if the node that smallest is set to has a student name of John well current is set to the same node so the student name of current is also John. Not an issue if they are different students with the same name but in your code they are the same student and both current and smallest point to the same node. Effectively this if statement is always going to be false and you will never execute the code to move current along the list. That is also why when you set smallest = last the method does work at least some of the time.

Answer 2

As the above said, try something like

smallest = startnode
next =startnode.next
while(next != null)
compare next with smallest, and assign accordingly
next = next.next

Shouldn't be too hard to turn this into code