下面是完整程序：
#include "stdio.h"
#include "malloc.h"
#define NULL 0
#define LEN sizeof(struct student)
struct student
{
    long num;
    int score;
    struct student *next;
};
struct student listA,listB;
int n,sum=0;
struct student *creat(void)
{
    struct student *p1,*p2,*head;
    n=0;
    p1=p2=(struct student *)malloc(LEN);
    printf("Input number&scores of student:\n");
    printf("If number is 0,stop inputing.\n");
    scanf("%ld,%d",&p1->num,&p1->score);
    head=NULL;
    while(p1->num!=0)
    {
        n=n+1;
        if(n==1)
        {
            head=p1;
        }
        else
        {
            p2->next=p1;
        }
        p2=p1;
        p1=(struct student *)malloc(LEN);
        scanf("%ld,%d",&p1->num,&p1->score);
    }
    p2->next=NULL;
    return(head);
}
struct student *insert(struct student *ah,struct student *bh)
{
    struct student *pa1,*pa2;
    struct student *pb1,*pb2;
    pa1=pa2=ah;
    pb1=pb2=bh;
    if(pa1->num<pb1->num)
    {
        ah=pa1;
        do
        {
            pa2=pa1;
            pa1=pa1->next;
        }while((pa1->num<pb1->num)&&(pa1->next!=NULL));
        if((pa1->num<pb1->num)&&(pa1->next==NULL))
        {
            pa1->next=pb1;
        }
        do
        {
            if(pa1->num>pb1->num)
            {
                pa2->next=pb1;
                do
                {
                    pb2=pb1;
                    pb1=pb1->next;
                }while((pa1->num>pb1->num)&&(pb1->next!=NULL));
                if((pa1->num>pb1->num)&&(pb1->next==NULL))
                {
                    pb1->next=pa1;
                }
            }
            if(pa1->num<pb1->num)
            {
                pb2->next=pa1;
                do
                {
                    pa2=pa1;
                    pa1=pa1->next;
                }while((pa1->num<pb1->num)&&(pa1->next!=NULL));
                if((pa1->num<pb1->num)&&(pa1->next==NULL))
                {
                    pa1->next=pb1;
                }
            }
        }while((pa1->next!=NULL)||((pa1->next==NULL)&&(pb1->next!=NULL)));
        if((pb1->num>pa1->num)&&(pa1->next==NULL))
        {
            pa1->next=pb1;
        }
    }
    if(pa1->num>pb1->num)
    {
        ah=pb1;
        do
        {
            pb2=pb1;
            pb1=pb1->next;
        }while((pa1->num>pb1->num)&&(pb1->next!=NULL));
        if((pa1->num>pb1->num)&&(pb1->next==NULL))
        {
            pb1->next=pa1;
        }
        do
        {
            if(pa1->num<pb1->num)
            {
                pb2->next=pa1;
                do
                {
                    pa2=pa1;
                    pa1=pa1->next;
                }while((pa1->num<pb1->num)&&(pa1->next!=NULL));
                if((pa1->num<pb1->num)&&(pa1->next==NULL))
                {
                    pa1->next=pb1;
                }
            }
            if(pa1->num>pb1->num)
            {
                pa2->next=pb1;
                do
                {
                    pb2=pb1;
                    pb1=pb1->next;
                }while((pa1->num>pb1->num)&&(pb1->next!=NULL));
                if((pa1->num>pb1->num)&&(pb1->next==NULL))
                {
                    pb1->next=pa1;
                }
            }
        }while((pb1->next!=NULL)||((pb1->next==NULL)&&(pa1->next!=NULL)));
        if((pa1->num>pb1->num)&&(pb1->next==NULL))
        {
            pb1->next=pa1;
        }
    }
    return(ah);
}
void print(struct student *head)
{
    struct student *p;
    printf("\nThere are %d records:\n",sum);
    p=head;
    if(p!=NULL)
    {
        do
        {
            printf("%ld %d\n",p->num,p->score);
            p=p->next;
        }while(p!=NULL);
    }
}
void main()
{
    struct student *creat(void);
    struct student *insert(struct student *,struct student *);
    void print(struct student *);
    struct student *ahead,*bhead,*abh;
    printf("Input list a:\n");
    ahead=creat();
    sum=sum+n;
    printf("Input list b:\n");
    bhead=creat();
    sum=sum+n;
    abh=insert(ahead,bhead);
    print(abh);
}
运行后提示应用程序错误：
"0x0040fe89"指令引用的“0x00000000"内存。该内存不能为"read"
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请问什么原因呢？请高手解答。

pa1=pa1->next;如果是最后一个数据话 ，赋值为0x0  就相当于pa1指向了一个空地址，你程序一运行 就发生段错误了

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