單鏈表到循環鏈表

Question

我一直在嘗試通過調整我的add（）和remove（）方法使單個鏈接列表變成一個雙循環列表。

這是我的代碼：

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();
}

在我的主要方法中：

TopSpinLinkedList<Integer> ll = new TopSpinLinkedList<Integer>(numTokens, spinSize);

    //fills LinkedList with tokens
    for(int i = 1; i <= numTokens; i++) {
        ll.add(i);
    }

當我嘗試調用此方法時：

//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;
}

在運行時出現NullPointerException。 我很確定這與我的add（）和remove（）方法有關。 我只是不明白我到底在做錯什么，使它成為一個雙向循環鏈接列表。 任何幫助將不勝感激。

Answer 1

在addToHead方法中，確定要在執行tail.next = newNode時初始化了tail變量嗎？

嘗試這個 ：

private void addToHead(E newItem)
    {
        LLNode<E> newNode = new LLNode<E>();
        newNode.data = newItem;
        head = newNode;
        tail = newNode;
        newNode.prev = newNode.next = newNode;
        size++;
    }

Answer 2

private void addToHead(E newItem){
    LLNode<E> newNode = new LLNode<E>();
    newNode.data = newItem;

    // head and tail are empty
    head = newNode;
    tail = newNode;
    head.next = newNode;
    tail.next = newNode;
    head.prev = newNode;
    tail.prev = newNode;
}


private void addAfter(E newItem, LLNode<E> where){
    if (where != null) {
        LLNode<E> newNode = new LLNode<E>();
        newNode.data = newItem;


        //this part is what you need
        newNode.next = where;
        newNode.prev = where.prev;
        where.prev.next = newNode; 
        where.prev = newNode;
        size++;
    } else {
        throw new NoSuchElementException();
    }
}

//我已經更改了代碼，現在應該可以了。 //我移位元件where一個位置向右，並在其位置插入newNode。

另外，如果您使用的是雙向循環鏈表，則不需要尾巴。 您可以簡單地擺脫它。

Answer 3

 template<class T> ostream& operator<<(ostream& os, CircularList<T>& ll) { if(!(ll.isEmpty())) { os<<"["; int size=ll.size(); int counting=0; while(counting<size) { os<<ll.get(counting); if(counting+1<size) os<<","; counting++; } os<<"]"; } else { os<<"[]"; } } template<class T> CircularList<T>::CircularList() { this->head = NULL; } template<class T> CircularList<T>::CircularList(const CircularList<T>& other) { this->head=0; Node<T>* tempHead=other.getLeader(); while(tempHead!=0) { Node<T> *temp = new Node<T>(tempHead->data); temp->next = 0; Node<T>* curr = this->getLeader(); if (curr != 0) { while (curr->next != 0) { curr = curr->next; } curr->next = temp; } else { this->head = temp; } tempHead=tempHead->next; } } template<class T> CircularList<T>& CircularList<T>::operator=(const CircularList<T>& other) { if(&other != this) { this->head=0; Node<T>* tempHead=other.head; while(tempHead!=0) { Node<T>* temp = new Node<T>(tempHead->data); temp->next = 0; Node<T>* curr = this->head; if (curr != 0) { while (curr->next != 0) { curr = curr->next; } curr->next = temp; } else { this->head = temp; } tempHead=tempHead->next; } } return *this; } template<class T> CircularList<T>* CircularList<T>::clone() { CircularList<T>* circle = new CircularList<T>(*this); } template<class T> CircularList<T>::~CircularList() { int count =size(); Node<T>* current = this->head; int i=0; while (i<count) { Node<T>* next = current->next; delete current; current = next; i++; } this->head = 0; } template<class T> void CircularList<T>::insert(int index, T data) { Node<T> *j= new Node<T>(data); if((0 <= index) &&( index<= size())) { if(index==0) { if(isEmpty()) { this->head=j; } else { Node<T>*skipping=this->head; this->head=j; this->head->next=skipping; } } else { Node<T> *tempping =this->head; int counting =1; while(counting!=index) { tempping=tempping->next; counting++; } j->next=tempping->next; tempping->next=j; } } else { throw ("invalid index"); } } template<class T> T CircularList<T>::remove(int index) { T pet; if(0 <= index && index<= size()-1) { if(!isEmpty()) { Node<T> *ret=getLeader(); Node<T>* skip=NULL; if(index!=0) { int i=1; while(i!=(index)) { ret=ret->next; i++; } skip=ret; pet=get(index); ret=ret->next; if(ret->next==NULL) { delete ret; skip->next=NULL; ret=NULL; } else { skip->next=ret->next; delete ret; ret=NULL; } } else { Node<T> *tmp = this->head->next; pet=get(index); delete this->head; this->head = tmp; } return pet; } else { throw ("Empty list"); } } else { throw ("Invalid index"); } } template<class T> T CircularList<T>::get(int index) const { if(this->head!=NULL) { if(0 <= index && index<= size()-1) { int counting=0; Node<T>* p =this->head; while(p!=NULL) { if(counting==index) { return p->data; } counting++; p=p->next; } } else { throw ("invalid index"); } } else { throw("empty list"); } } template<class T> bool CircularList<T>::isEmpty() { if(this->head==0) { return true ; } else { return false; } } template<class T> void CircularList<T>::clear() { Node<T>*serp=this->head; while(this->head!=NULL) { this->head=this->head->next; delete serp; serp=this->head; } } template<class T> Node<T>* CircularList<T>::getLeader() const { return this->head; } template<class T> ostream& CircularList<T>::print(ostream& os) { os<<*this; } template<class T> int CircularList<T>::size() const { int counting=0; Node<T> *serp =getLeader(); while(serp!=NULL) { serp=serp->next; counting++; } return counting; } template <class T> CircularList<T>& CircularList<T>::operator+(const CircularList<T>& other) { CircularList<T>* serp=new CircularList<T>(*this); Node<T>* hey=other.getLeader(); int counting=serp->size(); int position=0; while(hey!=NULL) { serp->insert(counting,other.get(position)); hey=hey->next; position++; counting++; } return *serp; } //a friend asked me to help him with some assignment i still have the code