双向链表的 C++ RemoveFront 函数

Question

我正在为学校开发一个实验室，但我被困在一个双向链表的函数上。当我尝试使用 removeFront() 函数从列表中删除前端节点时，它返回 NULL 和 couts 让我创建一个列表，即使已经有一个列表。我将在下面发布我的 cpp 文件和 main，希望有人可以帮助我了解它出了什么问题。

//DLinkedList.h

#pragma once

#include <string>
using namespace std;

typedef string Elem;

struct DNode
{ 
    Elem value;
    DNode* next;
    DNode* prev;
};

class DLinkedList
{
public:
    DLinkedList() { header_ = NULL; }
    ~DLinkedList() { };
    bool empty() const;
    const Elem& front() const;
    const Elem& back() const;
    void addFront(const Elem& e);
    void addBack(const Elem& e);
    void removeFront();
    void removeBack();

private:
    DNode* header_;
    DNode* trailer_;

protected:
    void add(DNode* v, const DNode& e);
    void remove(DNode* v);
};


//DLinkedList.cpp

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

using namespace std;



const Elem& DLinkedList::front() const
{
    if (header_ == NULL)
    {
        cout << "Please create a doubly linked list first.\n\n";
    }
    else
    {
        return header_->value;
    }

}

void DLinkedList::addFront(const Elem& e) //DONE
{
    // If there is no header
    // create a temporary node and set 
    // the header to it
    if (header_ == NULL)
    {
        DNode *temp;
        temp = new(struct DNode);
        temp->prev = NULL;
        temp->value = e;
        temp->next = NULL;

        header_ = temp;
        trailer_ = temp;
    }
    else
    {
        //Create current node to point to header
        // and temp node to be the new front node
        DNode *current;
        DNode *temp;
        current = header_;
        temp = new(struct DNode);
        temp->prev = NULL;
        temp->value = e;
        temp->next = current->next;

        header_->prev = temp->next;
        header_ = temp;
    }


    cout << "Element Inserted at the front." << endl;

}

void DLinkedList::removeFront()
{
    // Check to see if there is anything
    // in the list first
    if (header_ == NULL)
    {
        cout << "Create a doubly linked list first.";
    }

    // Check to see if the list has more than one item.
    // If it only has one node erase it.
    DNode *current;
    current = header_;

    if (current->next == NULL)
    {
        header_->next = NULL;
        header_->value = "";
        header_->prev = NULL;
        header_ = NULL;
    }
    else
    {
        current = current->next;
        //header_->next = NULL;
        //header_->value = "";
        //header_->prev = NULL;
        header_ = current;
        header_->prev = NULL;
    }
}

//Main.cpp

#include <iostream>
#include <string>
#include <cassert>
#include "DLinkedList.h"


using namespace std;

int main()
{
    DLinkedList album;
    album.addFront("Word");
    album.addFront("Turn");
    album.addFront("Bird");
    album.addFront("Weird");

    cout << album.front() << endl;

    album.removeFront();

    cout << album.front() << endl;


    system("pause");

    return 0;
}

Answer 1

我会这样做。

void DLinkedList::removeFront()
    {
        // Check to see if there is anything
        // in the list first
        if (header_ == NULL)
        {
            cout << "Create a doubly linked list first.";
        }

        // Check to see if the list has more than one item.
        // If it only has one node erase it.
        if (header_->next == NULL)
        {
            delete header_;
        }
        else
        {
            DNode* next = header_->next;
            delete header_;
            header_ = next;
            header_->prev = NULL;
        }
    }

Answer 2

我认为你的 addFront 也是错误的，我会这样做：

void DLinkedList::addFront(const Elem& content) //DONE
{
    // If there is no header
    // create a temporary node and set
    // the header to it
    if (header_ == NULL)
    {
        header_ = new(struct DNode);
        header_->prev = NULL;
        header_->value = content;
        header_->next = NULL;

        trailer_ = header_;
    }
    else
    {
        //Create current node to point to header
        // and temp node to be the new front node
        DNode *tmp;
        tmp = new(struct DNode);
        tmp->prev = NULL;
        tmp->value = content;
        tmp->next = header_;

        header_->prev = tmp;
        header_ = tmp;
    }


    cout << "Element Inserted at the front." << endl;

}