1.把雙鏈表這種資料結構的更高一級別抽象（線性表）單獨寫成一個抽象類，然後用於雙鏈表類的繼承，這種寫法可以規範雙鏈表應該有的介面/操作

2.把節點定義為類內的結構體而不是單獨的程式碼顯得更加簡潔。

3.在雙鏈表的插入/刪除/查詢操作中都要找到指向第i個節點的指標，把這個操作統一定義為一個私有的輔助函式move()，可以簡化程式碼

#include <iostream>
using namespace std;

// 線性表：順序表和連結串列共同的抽象類
template<typename elemType>
class list{
public:
    virtual void
 
 clear() = 0;                            // 刪除線性表中的所有資料元素
    virtual int length() const = 0;                      // 求線性表的長度
    virtual void insert(int i, const elemType & x) = 0;  // 在第i個位置插入一個元素
    virtual void remove(int i) = 0;                      // 刪除第i個位置的元素
    virtual int search(const elemType & x) const
 
 = 0;    // 搜尋某個元素x線上性表中是否出現
    virtual elemType visit(int i) const = 0;             // 返回線性表中第i個數據元素的值
    virtual void traverse() const = 0;                   // 按序訪問線性表的每一資料元素
    virtual ~list(){};
};


// 雙鏈表
template<typename elemType>
class dLinkList:public list<elemType>{
private:
    struct
 
 node{
        elemType data;
        node *prev, *next;

        node(const elemType & x, node* p = NULL, node* n = NULL)
            {data = x; prev = p; next = n;}
        node():next(NULL), prev(NULL){}
        ~node(){}
    };
    
    node *head, *tail;
    int currentLength;

    node* move(int i) const;

    class NoSuchNodeError{};

public:
    dLinkList();
    ~dLinkList(){clear(); delete head; delete tail;}

    void clear();
    int length() const{return currentLength;}
    void insert(int i, const elemType & x);
    void remove(int i);
    int search(const elemType & x) const;
    elemType visit(int i) const;
    void traverse() const;
};

template<typename elemType>
typename dLinkList<elemType>::node* dLinkList<elemType>::move(int i) const{
    node* p = head;

    while(i-- >= 0) p = p->next;
    return p;
}


template<typename elemType>
dLinkList<elemType>::dLinkList(){
    head = new node;
    tail = new node;
    head->next = tail;
    tail->prev = head;
    currentLength = 0;
}


template<typename elemType>
void dLinkList<elemType>::clear(){
    node* p = head->next;
    node* q;

    head->next = tail;
    tail->prev = head;
    while(p != tail){
        q = p->next;
        delete p;
        p = q;
    }
    currentLength = 0;
}


template<typename elemType>
void dLinkList<elemType>::insert(int i, const elemType & x){
    if(i > currentLength || i < 0) throw NoSuchNodeError();

    node* pos = move(i);
    node* tmp = new node(x, pos->prev, pos);

    pos->prev->next = tmp;
    pos->prev = tmp;
    ++currentLength;
}


template<typename elemType>
void dLinkList<elemType>::remove(int i){
    if(i >= currentLength || i < 0) throw NoSuchNodeError();

    node* pos = move(i);

    pos->prev->next = pos->next;
    pos->next->prev = pos->prev;
    delete pos;
    --currentLength;
}

template<typename elemType>
int dLinkList<elemType>::search(const elemType & x) const{
    node* p = head->next;
    int i;

    for(i = 0; p != tail && p->data != x; ++i) p = p->next;
    if(p == tail) return -1; else return i;
}

template<typename elemType>
elemType dLinkList<elemType>::visit(int i) const{
    if(i >= currentLength || i < 0) throw NoSuchNodeError();

    return move(i)->data;
}


template<typename elemType>
void dLinkList<elemType>::traverse() const{
    node* p = head->next;
    while(p != tail){
        cout << p->data << " ";
        p = p->next;
    }
    cout << endl;
}