带链表的 C++ 优先级队列类

Posted 2023-02-16

【中文标题】带链表的 C++ 优先级队列类

【英文标题】：C++ Priority Queue Class with Linked List

【发布时间】：2016-03-18 23:51:58

【问题描述】：

我的 c++ 代码有两个问题（测试文件如下）：

我似乎无法弄清楚为什么它没有跳出 while 循环，在运行时，它卡在循环“7 vs 325”上。

所以它应该进入 temp 的下一个节点，该节点将为 null，然后跳转到将其添加到队列末尾的部分。但它只是循环和循环。

我的第二个问题是我已注释掉的函数 queue1.back，无论何时运行，它都会出错并给出似乎是地址的内容，但 .front() 函数工作得很好。

我正在使用的测试文件是这样的：

89 Alex

325 Rob

72 Joy

91 Bob

using namespace std;

class Person

    friend class Pqueue;
    public:
        int priority;
        string name;
    ;


    class PQueue
    
        friend class Person;

        private:

        //Structure for my linked list.
        typedef struct node 
            Person data;
            struct node *next;
        Node, *NodePtr;

        Node *head, *tail;

        public:
        //Prototype Functions
        PQueue(void);              //Initializer function
        bool empty(void);          //Test if empty
        int size(void);            //Return size
        void enqueue(Person *);    //Insert Node
        void dequeue(void);        //Remove Node
        Person* front(void);       //Access Next Node
        Person* back(void);        //Access last node

    ;



    PQueue::PQueue()
   
       head = NULL;
       tail = NULL;
   



    bool PQueue::empty()
        return (head == NULL);
    



    void PQueue::enqueue(Person *myPerson)
        NodePtr np = (NodePtr) malloc(sizeof(Node));
        np->data = *myPerson;
        np->next = NULL;

        if(empty())
        
             cout << "Making into creating the first node, of the linked list" <<endl;
             head = np;
             tail = np;
         
         else  //Queue has more the one node
               Node* temp = head;
               if(np->data.priority > temp->data.priority) //If the priority is greater then the rest.
               
                   head = temp;                            //Saving my head pointer
                   head->data = np->data;                  //Assigning new Data to the head pointer
                   head->next = temp;                      //Assigning the rest of the linked list back into head.
                   cout << "Making into creating the first node again, having to reassign." <<endl;
               
               else
                    //Searching where to place the node.
                    while(temp->data.priority > np->data.priority) //Searching if the next priority is higher then the passed.
                    
                        cout << "Inside the while loop: " << np->data.priority << " versus "<<temp->data.priority <<endl;
                        if(temp->next == NULL)
                            break;
                        temp = temp->next;
                     

            if(temp->next == NULL && np->data.priority < temp->data.priority) //Inserting at the end.
            
                cout << "Making into creating the last node" <<endl;
                tail->next = np;
                cout << "Passing the function of creating the last node" <<endl;
            
            else   //Inserting into the middle of the function.
            
                cout << "Inserting in the middle of the queue" <<endl;
                np->next = temp->next;
                temp->next = np;
            
        
    




void PQueue::dequeue()
    if(empty())
        cout << "\nAttempt to remove from an empty list." << endl;
        exit(1);
    

    Person hold = head->data;
    NodePtr temp = head;
    head=head->next;
    if (head == NULL) tail = NULL;
    free(temp);


Person* PQueue::front()
    //Person &temp = head->next->data;
    //Person &temp = head->data;
    Person &temp = head->data;
    return &temp;


Person* PQueue::back()
    if(empty())
        cout << "\nNo entries in list." << endl;
        exit(1);
    
    Person &temp = tail->data;
    return &temp;


int main() 
    cout << "Starting main" << endl;
    PQueue queue1; //Creating my queue.
    cout << "Created Queue" << endl;
    Person tempPerson;
    ifstream inFile;
    inFile.open("/tmp/temp");
    cout << "going into while loop" << endl;

    while (inFile >> tempPerson.priority >> tempPerson.name)
        cout << "The priority is " <<  tempPerson.priority << " the name is " << tempPerson.name <<endl;
        queue1.enqueue(&tempPerson);
    


    //Testing Section, trying to get .front and .back to work.
    Person *testPerson;
    testPerson = queue1.front();
    cout << "The TEST priority is " <<  testPerson->priority << " the TEST name is " << testPerson->name <<endl;
    /**
    Person *tailPerson;
    testPerson = queue1.back();
    cout << "The TEST priority is " <<  tailPerson->priority << " the TEST  name is " << tailPerson->name <<endl;
    **/

    queue1.dequeue();
    queue1.dequeue();
    queue1.dequeue();


    return 0;

【问题讨论】：

仅供参考，您可能应该使用 new 运算符而不是 malloc。在 g++ 4.4.6 中，我遇到了段错误，大概是因为没有为 Person 类中的 name 字段调用构造函数。

【参考方案1】：

当您将新的头条目添加到非空列表时，您错误地将 next 指针设置为指向它所在的节点，而不是像您一样将其设置为指向链表的其余部分有意的。

head = temp;                            //Saving my head pointer
head->next = temp;                      //Assigning the rest of the linked list back into head.

【讨论】：

谢谢！这也是一个愚蠢的错误。啊。我认为问题出在循环上，因为它似乎被卡住了。我把它改成了head = nphead->next = temp。