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我在雙向鏈表中實現選擇排序。 我必須通過找到最小的元素並將其插入列表的開頭來按姓氏排序。 但是有一些麻煩，當我運行我的程序時，我在循環中的排序方法中有NIL異常。 這是整個應用程序，所以你可以編譯它並運行，如果你想。 幫助將不勝感激。 謝謝。選擇排序DLinked列表
public class LinkedList {
public Node first;
public Node last;
public LinkedList() {
first = null;
last = null;
}
public void addFirst(Student student) {
Node f = first;
Node newNode = new Node(student);
first = newNode;
if (f == null) last = newNode;
else {
f.previous = newNode;
newNode.next = f;
}
}
public void addLast(Student student) {
Node l = last;
Node newNode = new Node(student);
last = newNode;
if (l == null) first = newNode;
else {
l.next = newNode;
newNode.previous = l;
}
}
public void display() {
Node current = first;
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.next;
}
}
public Node findSmallest(Node startNode) {
Node smallest = startNode;
while (startNode.next != null) {
if (smallest.student.surname.compareToIgnoreCase(startNode.next.student.surname) > 1)
smallest = startNode.next;
else startNode = startNode.next;
}
return smallest;
}
public void Sort() {
LinkedList newList = new LinkedList();
Node current = first;
while (current.next != null) {
Node smallest = findSmallest(current);
newList.addLast(smallest.student);
delNode(smallest);
current = current.next;
}
first = newList.first;
last = newList.last;
}
public void delNode(Node toDel) {
if (toDel.previous == null) {
toDel.next.previous = null;
first = toDel.next;
return;
}
if (toDel.next == null) {
toDel.previous.next = null;
last = toDel.previous;
return;
}
toDel.previous.next = toDel.next;
toDel.next.previous = toDel.previous;
}
}
public class Student {
public String name;
public String surname;
public String educationType;
static public Student createStudent() {
Student student = new Student();
try {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
System.out.println("Enter student's name:");
student.name = br.readLine();
System.out.println("Enter surname:");
student.surname = br.readLine();
System.out.println("Enter educational type:");
student.educationType = br.readLine();
} catch (IOException e) {
throw new NotImplementedException();
}
return student;
}
}
public class Node {
public Student student;
public Node next;
public Node previous;
public Node(Student student) {
this.student = student;
}
}