关于链表的问题，如何解决？

Question

问题遇到的现象和发生背景
完成实验时，求一元多项式的乘积，遇到了一个bug

这是相关的代码

 //求多项式之间的乘积
 Polynomial MulPolyn(Polynomial Pa, Polynomial Pb)
 {
     int sum1 = 0;
     int sum2 = 0;
     //再建立一个链表
     Polynomial Pc = new PNode;
     Polynomial Pd = new PNode;
     Polynomial p1 = Pa->next;
     Polynomial p2 = Pb->next;
     Polynomial p3 = new PNode;
     Polynomial p4  = new PNode;
     //进行连接
     Pc->next = p3;
     Pd->next =  p4;
     //p3只需要一个节点来存放乘积
     //p4是最后的结果
     while (p1)
     {
         while (p2)
         {
             sum1 = p1->coef * p2->coef;
             sum2 = p1->expn + p2->expn;
             //将新生成的存放到链表C中
             p3->coef = sum1;
             p3->expn = sum2;
             //相当于add只加一个值
             AddPolyn(Pd,Pc);
             p2 = p2->next;
             Polynomial s = new PNode;
             p4->next = s;
             p4 = s;
         }
         p1 = p1->next;
     }
     return Pd;
 }

其中的加法函数

void AddPolyn(Polynomial& Pa, Polynomial& Pb)
{
    //p1,p2分别指向Pa.Pb的第一个节点
    Polynomial p1 = Pa->next;
    Polynomial p2 = Pb->next;
    //将p3指向p1
    Polynomial p3 = Pa;
    while (p1 && p2)
    {
        //总共存在三种情况，等于，大于和小于
        if (p1->expn == p2->expn)
        {
            double sum = p1->coef + p2->coef;
            if (sum == 0)
            {
                //分别删除p1,p2的第一个节点，并且移动向下一个节点
                Polynomial r1 = p1;
                p1 = p1->next;
                delete r1;
                Polynomial r2 = p2;
                p2 = p2->next;
                delete r2;
            }
            else
            {
                //将p1第一个节点中的指数修改为sum，然后删除p2中的当前节点，并且向下移动一位
                p1->coef = sum;
                //将当前节点指在p3后面
                p3->next = p1;
                p3 = p1;
                p1 = p1->next;
                //删除p2当前节点
                Polynomial r = p2;
                p2 = p2->next;
                delete r;
            }
        }
        else if (p1->expn < p2->expn)
        {
            p3->next = p1;
            p3 = p1;
            p1 = p1->next;
        }
        else
        {
            p3->next = p2;
            p3 = p2;
            p2 = p2->next;
        }
    }
    p3->next = p1 ? p1 : p2;
    delete Pb;
}

运行结果及报错内容
我的思路是：比如说（y1+y2）(y3+y4)，那么我在第一个括号中，每次循环取第一个，与第二个括号中进行相乘，依次得到各项结果.就结束了，我把pd用来存放改变,pc用来存放每一次的乘积，所以pc后面只需要跟一个链节，而pd在每一次的循环中增加一个链节，就相等于两个多项式相加了。
在我运行一遍后，会出现报错，访问权限发生冲突，会发生在加法的第二次循环中，按照我的想法，此时p1已将指空了，可是奇怪，在判定的时候又进去了，进去后，又认为不对了。
 
这里是全部的代码，希望看到的朋友可以帮忙检查一下，困扰好几个小时了。

#include<iostream>
using namespace std;
typedef int Status;
#define MAXSIZE 100
#define OK 1
#define ERROR 0


//链表的结构
typedef struct PNode {
    int coef;//系数
    int expn;//指数
    struct PNode* next;
}PNode,*Polynomial;

//多项式的创建
void CreatePolyn(Polynomial& P, int n)
{
    //有序链表P，多项式总共有n项
    //创建一个带有头节点的链表
    P = new PNode;
    P->next = NULL;
    for (int i = 1; i <= n; i++)
    {
        //创建新的节点
        Polynomial s = new PNode;
        //一会在主程序中提醒用户
        cin >> s->coef >> s->expn;
        //用来保存前驱
        Polynomial pre = P;
        //将指针q指向首元节点
        Polynomial q = P->next;
        while (q && q->expn < s->expn)
        {
            pre = q;
            q = q->next;
        }
        //后面连接
        s->next = q;
        //前面连接
        pre->next = s;
    }
}
void PrintPolyn(Polynomial P)
{
    Polynomial p1 = P->next;
    while (p1)
    {
        printf("%d(x)^%d ", p1->coef, p1->expn);
        if (p1->next)
            printf("+ ");
        p1 = p1->next;
    }
    printf("\n");
}
 void AddPolyn(Polynomial& Pa, Polynomial& Pb)
{
    //p1,p2分别指向Pa.Pb的第一个节点
    Polynomial p1 = Pa->next;
    Polynomial p2 = Pb->next;
    //将p3指向p1
    Polynomial p3 = Pa;
    while (p1 && p2)
    {
        //总共存在三种情况，等于，大于和小于
        if (p1->expn == p2->expn)
        {
            double sum = p1->coef + p2->coef;
            if (sum == 0)
            {
                //分别删除p1,p2的第一个节点，并且移动向下一个节点
                Polynomial r1 = p1;
                p1 = p1->next;
                delete r1;
                Polynomial r2 = p2;
                p2 = p2->next;
                delete r2;
            }
            else
            {
                //将p1第一个节点中的指数修改为sum，然后删除p2中的当前节点，并且向下移动一位
                p1->coef = sum;
                //将当前节点指在p3后面
                p3->next = p1;
                p3 = p1;
                p1 = p1->next;
                //删除p2当前节点
                Polynomial r = p2;
                p2 = p2->next;
                delete r;
            }
        }
        else if (p1->expn < p2->expn)
        {
            p3->next = p1;
            p3 = p1;
            p1 = p1->next;
        }
        else
        {
            p3->next = p2;
            p3 = p2;
            p2 = p2->next;
        }
    }
    p3->next = p1 ? p1 : p2;
    delete Pb;
}
 //调用减法
 void MinusPolyn(Polynomial& Pa, Polynomial& Pb)
 {
     //加一个负的就好
     Polynomial p = Pb->next;
     while (p)
     {
         p->coef = -(p->coef);
         p = p->next;
     }
     AddPolyn(Pa, Pb);
 }
 //计算多项式在X=X0处的值
 double ResultPolyn(Polynomial Pa)
 {
     int sum = 0;
     int x0;
     int result;
     cin >> x0;
     Polynomial p = Pa->next;
     while (p)
     {
         result = (p->coef) * pow( x0, p->expn);
         sum += result;
         p = p->next;
     }
     return sum;
 }
 //求多项式的导数
 Status DerPolyn(Polynomial Pa)
 {
     Polynomial p = Pa->next;
     while (p)
     {
         if (p->coef < 0 && p->expn == 0.5)
         {
             return ERROR;
         }
         p->coef = (p->coef) * (p->expn);
         p->expn = (p->expn) - 1;
         p = p->next;
     }
     PrintPolyn(Pa);
     return OK;
 }
 //求多项式之间的乘积
 Polynomial MulPolyn(Polynomial Pa, Polynomial Pb)
 {
     int sum1 = 0;
     int sum2 = 0;
     //再建立一个链表
     Polynomial Pc = new PNode;
     Polynomial Pd = new PNode;
     Polynomial p1 = Pa->next;
     Polynomial p2 = Pb->next;
     Polynomial p3 = new PNode;
     Polynomial p4  = new PNode;
     //进行连接
     Pc->next = p3;
     Pd->next =  p4;
     //p3只需要一个节点来存放乘积
     //p4是最后的结果
     while (p1)
     {
         while (p2)
         {
             sum1 = p1->coef * p2->coef;
             sum2 = p1->expn + p2->expn;
             //将新生成的存放到链表C中
             p3->coef = sum1;
             p3->expn = sum2;
             //相当于add只加一个值
             AddPolyn(Pd,Pc);
             p2 = p2->next;
             Polynomial s = new PNode;
             p4->next = s;
             p4 = s;
         }
         p1 = p1->next;
     }
     return Pd;
 }
 
int main()
{
    void test1();
    void test2();
    test1();
    test2();
    return 0;
}


void test1()
{
    menu();
    Polynomial s1;
    Polynomial s2;
    CreatePolyn(s1, 2);
    PrintPolyn(s1);
    CreatePolyn(s2, 2);
    PrintPolyn(s2);
    //MinusPolyn(s1, s2);
    //PrintPolyn(s1);
    /*AddPolyn(s1, s2);*/
    //PrintPolyn(s1);
    int result = ResultPolyn(s1);
    cout << "结果是：" << result<<endl;
    DerPolyn(s1);
    Polynomial s3 = MulPolyn(s1, s2);
    PrintPolyn(s3);
}

Answer 1

举个例子

#include <iostream>
using namespace std;
#define M 3
#define N 2
#define S (M + N - 1)
int main(int argc, char* argv[])
{
    int a[M] = {2, 0, 1};
    int b[N] = {5, -1};
    int c[S] = {0};
    // 输出乘法因子
    cout<<"输入多项式为："<<endl;
    for (int i=0; i<M; i++)
    {
        cout<<a[i]<<'/t';
    }
    cout<<endl;
    for (i=0; i<N; i++)
    {
        cout<<b[i]<<'/t';
    }
    cout<<endl;
    // 进行乘法运算
    for (i=0; i<M; i++)
    {
        for (int j=0; j<N; j++)
        {
            c[i+j] += a[i] * b[j];
        }
    }
    // 输出乘法结果
    cout<<"输出多项式为："<<endl;
    for (i=0; i<S; i++)
    {
        cout<<c[i]<<'/t';
    }
    cout<<endl;
    return 0;
}