输入两个链表a,b，包括学号、成绩。要求把两个链表合并，按学号升序排列；现象是，输入a,b链表正常，但是合并链表(链表c)却是NULL,错误初步认定在insert()函数里面，帮忙看看~~谢谢啦o(∩_∩)o

#include<stdio.h>
#include<malloc.h>
typedef struct student
{
int code,score;
struct student* next;
} ST;
main()
{  
    ST *creat();
    ST *insert();
    ST *ahead,*bhead,*chead;
    void pr();
    printf("输入A链表数据：\n");
    ahead=creat();
    printf("输入B链表数据：\n");
    bhead=creat();
    printf("\n");
    printf("A链表：\n");
    pr(ahead);
    printf("---------------------------------\n");
    printf("B链表：\n");
    pr(bhead);
    chead=insert(ahead,bhead);
    printf("---------------------------------\n");
    printf("C链表：\n");
    pr(chead);
}
ST *creat(void)
{
ST *p1,*p2,*head;
head=(ST *)malloc(sizeof (ST));
p1=head;
p1->next=NULL;
p2=(ST *)malloc(sizeof (ST));
printf("输入学号,成绩：\n");
scanf("%d,%d",&p2->code,&p2->score);
while(p2->code!=0)
{   
   p1->next=p2;
      p1=p2;
   p2=(ST *)malloc(sizeof (ST));
   printf("输入学号,成绩：\n");
   scanf("%d,%d",&p2->code,&p2->score);
   p1->next=NULL;
}
return (head);
}


void pr(ST *head)
{   
ST *p;
p=head;
p=p->next;
if (p)
do
{   
   printf("\ncode:%d\nscore:%d\n",p->code,p->score);
   p=p->next;
}while (p);
}


ST *insert(ST *ahead,ST *bhead)
{
ST *p1,*p2,*p,*chead;
p1=ahead->next;
p2=bhead->next;
p=chead=(ST *)malloc(sizeof (ST));
p->next=NULL;
while(p1 && p2)
{
   if(p1->code>p2->code)
   {
    p->next=p2;
    p2=p2->next;
    p->next=NULL;
   }
   else
   {
    p->next=p1;
    p1=p1->next;
    p->next=NULL;
   }
}
     if(p1=NULL) p->next=p2;
else p->next=p1;
return (chead);

}