我正在为下周的考试而学习，我希望在添加和删除双向链接列表中的元素时，有人可以帮助我。这个逻辑对我来说很有意义，不幸的是我在编码方面还不是很好（只有我的第二个计算机科学课程）。我已经浏览了整个网络和我的文本，但我似乎无法找到一个足够接近我的问题的例子。下面我将发布我的教授要求我完成的内容，以及我目前的代码。提前感谢您的帮助。
更新
我在添加时遇到了问题，但是有人能够帮助我，但是我的删除方法仍然存在问题。我觉得奇怪的是我的教授有一个除了void之外的返回类型的remove方法。有人可以解释一下吗？无论如何，我的更新代码如下。
来自教授：
将CS401DblLinkedListImpl.java中缺少的代码填入 在课堂上讲了。
要测试您的代码，请创建一个双向链接列表并输入 其中包含String类型的元素：
比尔，罗汉，詹姆斯，克里希纳，哈维尔，丽莎（a）从头开始打印链表。
（b）从最后开始打印链表。
（c）删除账单并从头开始打印链表。
（d）删除Lisa并从头开始打印链表。
（e）删除Krishna并从中打印链接列表 开始。
以下代码集是我的用户定义类，我将测试的方法是boolean add（E e），E remove（int n），void print_from_beginning（）和void print_from_end（）：
package week6;
import java.util.Iterator;
public class CS401DblLinkedListImpl<E> //implements CS401CollectionInterface<E>
{
private LinkEntry<E> head = null;
private LinkEntry<E> tail = null;
private int size = 0;
public CS401DblLinkedListImpl()
{
head = tail = null;
}
public boolean is_empty()
{
if (head == null)
return true;
return false;
}
public int size()
{
int count = 0;
for (LinkEntry<E> current = head; current != null; current = current.next)
count++;
return count;
}
/*
* Add e to the end of the doubly linked list.
* Returns true - if e was successfully added, false otherwise.
*/
public boolean add(E e)
{
LinkEntry<E> new_element = new LinkEntry<E>();
new_element.element = e;
if (head == null)
{
new_element.next = head;
head = new_element;
tail = head;
}
else
{
tail.next = new_element;
new_element.previous = tail;
tail = new_element;
}
return true;
}
/*
* Remove the nth element in the list. The first element is element 1.
* Return the removed element to the caller.
*/
public E remove(int n)
{
LinkEntry<E> current = new LinkEntry<E>();
int i = 0;
while (n == i++)
{
current.previous.next = current.next;
if (current.next == null)
{
current.next.previous = current.previous;
}
}
return (E) current;
}
/*
* Print the doubly linked list starting at the beginning.
*/
public void print_from_beginning()
{
LinkEntry<E> current = new LinkEntry<E>();
for (current = head; current != null; current = current.next)
{
System.out.print(current.element + " ");
}
}
/*
* Print the doubly linked list starting the end.
*/
public void print_from_end()
{
LinkEntry<E> current = new LinkEntry<E>();
for (current = tail; current != null; current = current.previous)
{
System.out.print(current.element + " ");
}
}
/* ------------------------------------------------------------------- */
/* Inner classes */
protected class LinkEntry<E>
{
protected E element;
protected LinkEntry<E> next;
protected LinkEntry<E> previous;
protected LinkEntry() { element = null; next = previous = null; }
}
/* ------------------------------------------------------------------- */
protected class CS401DblLinkedListImplIterate<E> implements Iterator<E>
{
protected LinkEntry<E> next;
protected CS401DblLinkedListImplIterate()
{
next = (LinkEntry<E>) head;
}
@Override
public boolean hasNext() {
// TODO Auto-generated method stub
return false;
}
@Override
public E next() {
// TODO Auto-generated method stub
return null;
}
@Override
public void remove() {
// TODO Auto-generated method stub
}
}
} /* CS401LinkedListImpl<E> */
下面是我测试方法的主要课程：
package week6;
import java.util.*;
public class App {
public static <E> void main(String[] args) {
/*
* Part 1 of lab 6: Testing CS401DblLinkedListImpl
*/
System.out.println("Testing Lab 6: Part 1...");
CS401DblLinkedListImpl<String> list = new CS401DblLinkedListImpl<String>();
list.add("Bill");
list.add("Rohan");
list.add("James");
list.add("Krishna");
list.add("Javier");
list.add("Lisa");
System.out.println("List size after all names are added: " + list.size());
//a. Print the linked list starting at the beginning.
System.out.println("\nPrint the linked list starting at the beginning:");
list.print_from_beginning();
//b. Print the linked list starting at the end.
System.out.println("\nPrint the linked list starting at the end:");
list.print_from_end();
//c. Remove Bill and print the linked list starting from beginning.
System.out.println("\nRemove Bill and print the linked list starting from beginning:");
list.remove(0);
list.print_from_beginning();
//d. Remove Lisa and print the linked list starting from end.
System.out.println("\nRemove Lisa and print the linked list starting from end:");
list.remove(4);
list.print_from_end();
//e. Remove Krishna and print the linked list starting from the beginning.
System.out.println("\nRemove Krishna and print the linked list starting from the beginning:");
list.remove(2);
list.print_from_beginning();
System.out.println("\nList size: " + list.size());
}
}
最后，下面是我运行程序时得到的结果：
Testing Lab 6: Part 1...
List size after all names are added: 6
Print the linked list starting at the beginning:
Bill Rohan James Krishna Javier Lisa
Print the linked list starting at the end:
Lisa Javier Krishna James Rohan Bill
Remove Bill and print the linked list starting from beginning:
Exception in thread "main" java.lang.NullPointerException
at week6.CS401DblLinkedListImpl.remove(CS401DblLinkedListImpl.java:67)
at week6.App.main(App.java:34)
仅供参考，CS401DblLinkedListImpl.java：67在我的remove方法中引用以下代码行：
current.previous.next = current.next;
对此的任何帮助将不胜感激。我觉得我有点接近答案，只需要澄清一下。感谢。
答案 0 :(得分：1)
您当前的代码无法处理删除索引0。
LinkEntry<E> current = head;
if (n == 0) {
// something like this maybe...
head = head.next;
if (head != null) { head.previous = null; }
return current;
}
你从i = 0开始，但是做一个i ++，所以你处于领先地位，但索引为1。
你的删除逻辑对我来说也很奇怪。
current.previous.next == current (is true)
current.next.previous == current (is true)
所以它应该是这样的：
if (n == i++) {
// We're at the spot so let's remove it.
current.previous.next = current.next;
if (current.next != null) { current.next.previous = current.previous; }
return current;
}
另外我相信你添加逻辑也是错误的，应该是这样。
LinkEntry<E> new_element = new LinkEntry<E>();
new_element.element = e;
if (head == null) {
head = new_element;
tail = head;
} else {
tail.next = new_element;
new_element.previous = tail;
tail = new_element;
}
size++; // You don't use size it looks like... and your size starts at 1 which is
// wrong, it should start at 0 since it's empty, also remove would have to
// update size.
return true;