我正在编写一个通用数据结构，它可以从我测试代码的第一个或最后一个节点中添加和删除，但是我在某些特定的输入方式中遇到了异常。
现在如果我 addlast 然后 addfirst 然后 removelast 我得到了异常
当我 addfirst 很多次而不添加 last 然后尝试通过 removelast() 函数删除它们我得到了异常但是当我 addlast 很多次而不首先添加然后通过 removefirst() 删除它们它可以工作
我试图避免while循环这里是代码
import java.util.Iterator;
public class Deque <Item> implements Iterable <Item> {
private Node first,last;
private class Node
{
Item item;
Node next;
Node prev;
}
public Deque()
{
first = null;
last = null;
}
public boolean IsEmpty()
{
return first == null;
}
public void addFirst(Item item)
{
Node oldfirst = first;
first = new Node();
first.item = item;
first.next = oldfirst;
first.prev = null;
if (last == null)
{
last = first;
}
}
public void addlast(Item item)
{
Node oldlast = last;
last = new Node();
last.item = item;
last.next = null;
if (first == null)
{
first = last;
}
else
{
last.prev = oldlast;
oldlast.next = last;
}
}
public Item removeFirst()
{
Item x = first.item;
first = first.next;
if (IsEmpty())
last = null;
return x;
}
public Item removeLast()
{
if (first == last)
return removeFirst();
Item x = last.item;
last = last.prev;
last.next = null;
if (IsEmpty())
first = null;
return x;
}
public Iterator<Item> iterator ()
{
return new ListIterator();
}
private class ListIterator implements Iterator<Item>
{
private Node current = first;
public boolean hasNext ()
{
return current != null;
}
public void remove()
{
//NOt Supported
}
public Item next()
{
Item x = current.item;
current = current.next;
return x;
}
} }
我相信我在 removelast() 中的 last.prev 有问题，因为它已经为空，然后在 remove() 中引用了 last = last.perv 但我想不出一种方法将 last 链接到 first 的最后一个节点
谁能帮我这个
如果你想尝试，这里是主要的......
public class Main {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Deque<Integer> o = new Deque<Integer>();
int num = 0;
while (true)
{
StdOut.println("enter 1 to addfirst, 2 addlast, 3 removefirst, 4 removelast, " +
"5 to exit");
num = StdIn.readInt();
if (num == 5)
break;
switch (num)
{
case 1:
StdOut.println("enter number to add first");
int x = StdIn.readInt();
o.addFirst(x);
break;
case 2:
StdOut.println("enter number to add last");
int y = StdIn.readInt();
o.addlast(y);
break;
case 3:
int w=o.removeFirst();
StdOut.print("the deleted number is: ");
StdOut.print(w);
StdOut.println();
break;
case 4:
int z=o.removeLast();
StdOut.print("the deleted number is: ");
StdOut.print(z);
StdOut.println();
break;
default:
StdOut.println("Stick with the range!");
break;
}
for (Iterator<Integer> i=o.iterator(); i.hasNext();)
{
StdOut.print(i.next());
StdOut.print(" ");
}
StdOut.println();
}
}
}