第三十三课 双向循环链表的实现

头结点不位于链表里面，只是用于定位，和内核链表不同。

 

将LinuxList.h添加到我们的工程中。

再添加一个DualCircleList.h文件：

#ifndef DUALCIRCLELIST_H
#define DUALCIRCLELIST_H

#include "LinuxList.h"
#include "DualLinkList.h"

namespace DTLib
{

template < typename T >
class DualCircleList : public DualLinkList<T>
{
protected:
    struct Node : public Object
    {
        list_head head;
        T value;
    };

    list_head m_header;
    list_head* m_current;

    list_head* position(int i) const
    {
        list_head* ret = const_cast<list_head*>(&m_header); //这里需要进行const_cast转换，否则无法在const函数里面取m_header的地址

        for(int p=0; p<i; p++)
        {
            ret = ret->next;
        }

        return ret;
    }

    int mod(int i) const
    {
        return (this->m_length == 0) ? 0 : (i % this->m_length);;
    }

public:
    DualCircleList()
    {
        this->m_length = 0;
        this->m_step = 1;

        m_current = NULL;

        INIT_LIST_HEAD(&m_header);
    }

    bool insert(const T& e)
    {
        return insert(this->m_length, e);
    }

    bool insert(int i, const T& e)
    {
        bool ret = true;
        Node* node = new Node();

        i = i % (this->m_length + 1);

        if(node != NULL)
        {
            node->value = e;

            list_add_tail(&node->head, position(i)->next);

            this->m_length++;
        }
        else
        {
            THROW_EXCEPTION(NoEnoughMemoryException, "No memory to create Node object ...");
        }

        return ret;
    }

    bool remove(int i)
    {
        bool ret = true;

        i = mod(i);

        ret = ((0 <= i) &&(i<this->m_length));

        if( ret )
        {
            list_head* toDel = position(i)->next;

            if( m_current == toDel )
            {
                m_current = toDel->next;
            }

            list_del(toDel);

            this->m_length--;

            delete list_entry(toDel, Node, head);
        }

        return ret;
    }

    bool set(int i, const T& e)   //  O(n)
    {
        bool ret = true;
        i = mod(i);
        ret = ((0 <= i) && (i < this->m_length));

        if( ret )
        {
            list_entry(position(i)->next, Node, head)->value = e;
        }

        return ret;
    }

    virtual T get(int i) const   // O(n)
    {
        T ret;

        if( get(i, ret) )
        {
            return ret;
        }
        else
        {
            THROW_EXCEPTION(IndexOutOfBoundsException, "Invalid parameter i to get element ...");
        }

        return ret;
    }

    bool get(int i, T& e) const    // O(n)
    {
        bool ret = true;

        i = mod(i);

        ret = ((0 <= i) && (i < this->m_length));

        if( ret )
        {
            e = list_entry(position(i)->next, Node, head)->value;
        }

        return ret;
    }

    int find(const T& e) const
    {
        int ret = -1;

        int i = 0;

        list_head* slider = NULL;

        list_for_each(slider, &m_header)
        {
            if( list_entry(slider, Node, head )->value == e)
            {
                ret = i;
                break;
            }

            i++;
        }

        return ret;
    }

    int length() const
    {
        return this->m_length;
    }

    void clear()
    {
        while( this->m_length > 0 )
        {
            remove(0);
        }
    }

    bool move(int i, int step = 1)
    {
        bool ret = (step > 0);

        i = mod(i);

        ret = ret && ((0 <= i) && (i < this->m_length));

        if( ret )
        {
            m_current = position(i)->next;

            this->m_step = step;
        }

        return ret;
    }

    bool end()
    {
        return (m_current == NULL) || (this->m_length == 0);
    }

    T current()
    {
        if( !end() )
        {
            return list_entry(m_current, Node, head)->value;
        }
        else
        {
            THROW_EXCEPTION(InvalidOperationException, "No value at current position ...");
        }
    }

    bool next()
    {
        int i = 0;

        while((i < this->m_step) && !end())
        {
            if( m_current != &m_header )
            {
                m_current = m_current->next;
                i++;
            }
            else
            {
                m_current = m_current->next;
            }
        }

        if( m_current == &m_header )
        {
            m_current = m_current->next;
        }

        return (i == this->m_step);
    }

    bool pre()
    {
        int i = 0;

        while((i < this->m_step) && !end())
        {
            if( m_current != &m_header )
            {
                m_current = m_current->prev;
                i++;
            }
            else
            {
                m_current = m_current->prev;
            }
        }

        if( m_current == &m_header )
        {
            m_current = m_current->prev;
        }

        return (i == this->m_step);
    }

    ~DualCircleList()
    {
        clear();
    }
};

}

#endif // DUALCIRCLELIST_H

 我们的实现中，头结点只为了做定位用，不属于链表中的一个元素，因此，next和pre函数要跳过头结点。

单向循环链表CircleList是继承的LinkList的next函数，只向一个方向移动，而且不会越过头结点。

测试程序如下：

#include <iostream>
#include "DualCircleList.h"

using namespace std;
using namespace DTLib;


int main()
{
    DualCircleList<int> dl;

    for(int i = 0; i < 5; i++)
    {
        dl.insert(0, i);
        dl.insert(0, 5);
    }

    cout << "begin" << endl;

    dl.move(dl.length() - 1);

    while(dl.find(5) != -1)   //所有的5全部被删除后停止循环
    {
        if( dl.current() == 5 )
        {
            cout << dl.current() << endl;

            dl.remove(dl.find(dl.current()));
        }
        else
        {
            dl.pre();
        }
    }

    cout << "end" << endl;

    for(int i = 0; i < dl.length(); i++)
    {
        cout << dl.get(i) << endl;
    }

    return 0;
}

运行结果如下：

使用以下程序访问会得到死循环：

 因为当前是一个双向循环链表。

小结：

思考题：