单链表实现

template <class Elem> class Link {public:    Elem element;    Link *next;    Link(const Elem& elemval, Link* nextval = NULL) {        element = elemval;        next = nextval;    }    Link(Link* nextval = NULL) { next = nextval; }};template <class Elem> class LList: public List<Elem> {private:    Link<Elem>* head;    Link<Elem>* tail;    Link<Elem>* fence;    int leftcnt;    int rightcnt;    void init() {        fence = tail = head = new Link<Elem>;        leftcnt = rightcnt = 0;    }    void removeall() {        while (head != NULL) {            fence = head;            head = head->next;            delete fence;        }    }public:    LList(int size=DefaultListSize) { init(); }    ~LList() { removeAll(); }    void clear() { removeAll(); init(); }    bool insert(const Elem&);    bool append(const Elem&);    bool remove(Elem&);    void setStart() {        fence = head;        rightcnt += leftcnt;        leftcnt = 0;    }    void setEnd() {        fence = tail;        leftcnt += rightcnt;        rightcnt = 0;    }    void prev();    void next() {        if (fence != tail) {            fence = fence->next;            rightcnt--;            leftcnt++;        }    }    int leftLength() const { return leftcnt; }    int rightLength() const { return rightcnt; // rightcnt是fence右边的元素个数}    bool setPos(int pos);    bool getValue(Elem& it) const {        if (rightLength() == 0) return false;        it = fence->next->element; // fence指向的是栅栏左边的最后一个元素        return true;    }    void print() const;};template <class Elem>bool LList<Elem>::insert(const Elem& item) {    fence->next = new Link<Elem>(item, fence->next);    if (tail == fence) tail = fence->next;    rightcnt++;    return true;}template <class Elem>bool LList<Elem>::append(const Elem& item) {    tail = tail->next = new Link<Elem>(item, NULL);    rightcnt++;    return true;}template <class Elem> bool LList<Elem>::remove(Elem& it) {    if (fence->next == NULL) return false;    it = fence->next->element;    Link<Elem>* ltemp = fence->next;    fence->next = ltemp->next;    if (tail == ltemp) tail = fence;    delete ltemp;    rightcnt--;    return true;}template <class Elem> void LList<Elem>::prev() {    Link<Elem>* temp = head;    if (fence == head) return;    while (temp->next != fence) temp = temp->next;    fence = temp;    leftcnt--;    rightcnt++;}template <class Elem> bool LList<Elem>::setPos(int pos) {    if ((pos < 0) || (pos > rightcnt + leftcnt)) return false;    fence = head;    for (int i = 0; i<pos; ++i) fence = fence->next;    return true;}template <class Elem> void LList<Elem>::print() const {    Link<Elem>* temp = head;    cout<<"< ";    while (temp != fence) {        cout << temp->next->element << " ";        temp = temp->next;    }    cout<< "| ";    while (temp->next != NULL) {        cout << temp->next->element << " ";        temp = temp->next;    }    cout << ">\n"; }