我试着写一些伪语句来找出问题所在。问题似乎出现在merge()函数的末尾，但我不知道问题出在哪里。

#include <iostream>
using namespace std;

int value, count, value2, count2,choice;

struct node
{
    int data;
    node * next;
};

node *list = nullptr;
node *list2 = nullptr;
node * p;
node * q;
node * r;

void insertFront()
{
    cout << "ENTER A VALUE=";
    cin >> value;

    if (list == nullptr)
    {
        p = new node();
        p->data = value;
        p->next = nullptr;
        list = p;
    }
    else
    {
        p = new node();
        p->data = value;
        p->next = list;
        list = p;
    }
}

void insertFront2()
{
    cout << "ENTER A VALUE=";
    cin >> value;

    if (list2 == nullptr)
    {
        r = new node();
        r->data = value;
        r->next = nullptr;
        list2 = r;
    }
    else
    {
        r = new node();
        r->data = value;
        r->next = list2;
        list2 = r;
    }
}

void delFront()
{
    if (list == nullptr)
    {
        cout << "LIST IS ALREADY EMPTY";
    }
    else
    {
        p = list;
        list = p->next;
        delete(p);
    }
}

void display(int choice)
{
    int select=choice;

    if (select == 1)
    {
        p = list;
    }
    if (select == 2)
    {
        p = list2;
    }
    while (p != nullptr)
    {
        cout << p->data << " ";
        p = p->next;
    }

    cout << endl;
}

void mergeLists(node *list,node *list2)
{
    if (list==nullptr and list2==nullptr){
        cout<<"Both lists are empty";
    }

    else if(list==nullptr && list2!=nullptr){
        display(2);
    }

    else if(list2==nullptr && list!=nullptr){
        display(1);
    }
    if(list!=nullptr && list2!=nullptr){
        p=list;
        while(p!=nullptr){
            p=p->next;
        }

        p->next=list2;

        display(1);
    }
}

int main()
{
    int choice;
    cout << "1) Insert at front " << endl;
    cout << "2) Delete at front" << endl;
    cout << "7) Merge two lists" << endl;
    cout << "9) Display" << endl << endl;
    while (choice != 99)
    {
        cout << "Your choice:";
        cin >> choice;

        switch (choice)
        {
            case 1:
                {
                    int sel;
                    cout << "Enter the list to which you want to enter:\n 1 or 2\nYour choice:";
                    cin >> sel;
                    if (sel == 1)
                    {
                        insertFront();
                    }
                    else if (sel == 2)
                    {
                        insertFront2();
                    }

                    break;
                }

            case 2:
                {
                    delFront();
                    break;
                }

            case 7:
                {
                    mergeLists(list,list2);

                    break;
                }

            case 9:
                {
                    int sel;
                    cout<<"Select a list to display: 1/2"<<endl;
                    cin>>sel;
                    display(sel);
                    break;
                }

            case 99:
                {
                    cout << "PROGRAM TERMINATED :)";
                    break;
                }
        }
    }

    return 0;
}