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我一直试图通过调整我的add（）和remove（）方法，使我的单链表成为双圆形。单链表到循环链表
这里是我的代码：
private LLNode<E> head; // the first node in the list
private LLNode<E> tail; // the last node in the list
// Adds the specified new item to the back of the list.
public void add(E newData)
{
if (head == null) // if the list is empty...
addToHead(newData);
else
addAfter(newData, nodeAt(size - 1));
}
// Removes and returns the item at the specified index of the list.
public E remove(int index)
{
if (index == 0) // if removing from the head...
return removeFromHead();
else
return removeAfter(nodeAt(index - 1));
}
private void addToHead(E newItem)
{
LLNode<E> newNode = new LLNode<E>();
newNode.data = newItem;
newNode.next = head;
head = newNode;
head.prev = tail;
tail.next = newNode;
size++;
}
// Removes the head node from the list.
private E removeFromHead()
{
if (head != null) {
E temp = head.data;
head = head.next;
head.prev = tail;
tail.next = head;
size--;
return temp;
} else
throw new NoSuchElementException();
}
// Adds a new node containing the specified data, after the
// specified node in the list.
private void addAfter(E newItem, LLNode<E> where)
{
if (where != null) {
LLNode<E> newNode = new LLNode<E>();
newNode.data = newItem;
newNode.next = where.next;
where.next = newNode;
newNode.prev = where;
size++;
} else {
throw new NoSuchElementException();
}
}
// Removes the node after the specified node in the list.
private E removeAfter(LLNode<E> where)
{
if (where != null && where.next != null) {
E temp = where.next.data;
where.next = where.next.next;
where.next.prev = where;
size--;
return temp;
} else
throw new NoSuchElementException();
}
在我的主要方法：
TopSpinLinkedList<Integer> ll = new TopSpinLinkedList<Integer>(numTokens, spinSize);
//fills LinkedList with tokens
for(int i = 1; i <= numTokens; i++) {
ll.add(i);
}
当我尝试调用这个方法：
//shifts all elements in LinkedList to left by one
public void shiftLeft()
{
if(head == null || head.next == null)
return;
LLNode<E> temp = new LLNode<E>();
temp = head;
head = head.next;
temp.next = null;
tail.next = temp;
tail = temp;
}
一个NullPointerException异常运行时显示。我很确定它与我的add（）和remove（）方法有关。我只是不明白我到底做了什么错误，把它变成一个双向链表。任何帮助将非常感激。
在 “tail.next = newNode;”在addToHead（）方法 – GreatBambino