这个是求两个多项式之和，第一个多项式的数据输入之和按回车程序就运行出错了，求大神解答！

#include<stdio.h>
#include<stdlib.h>
struct node{int coef;
            int exp;
            struct node*link;
           };
typedef struct node NODE;
NODE *ah,*bh,*ch;
NODE*creat(void)
{
    char coef,exp;
    int n=0;
    NODE*head,*p,*q;
    do
    {
        scanf("%c%c",&coef,&exp);
        if(coef==0);
        break;
        n++;
        p=(NODE*)malloc(sizeof(NODE));
        p->coef=coef,p->exp=exp,p->link=NULL;
        if(n==1)
        head=p=q;
        else
        {
            q->link=p;
            q=p;
        }
    }while(1);
    return head;
}
NODE*insert(NODE*pc,int c,int e)
{
    NODE *t;
    t=(NODE*)malloc(sizeof(NODE));
    t->coef=c;
    t->exp=e;
    pc->link=t;
    return t;
}
NODE *polyadd_l(ah,bh)
 NODE *ah,*bh;
 {
     NODE*pa,*pb,*ch,*pc;
     char c;
     ch=(NODE*)malloc(sizeof(NODE));
     pc=ch;
     pa=ah;
     pb=bh;
     while(pa!=NULL&&pb!=NULL)
     {
         if(pa->exp==pb->exp)c='=';
         else if(pa->exp>pb->exp)c='>';
         else c='<';
         switch(c)
         {
             case'=':
             if(pa->coef+pb->coef!=0)
             pc=insert(pc,pa->coef+pb->coef,pa->exp);
             pa=pa->link;
             pb=pb->link;
             break;
             case'>':
             pc=insert(pc,pa->coef,pa->exp);
             pa=pa->link;
             break;
             case'<':
             pc=insert(pc,pb->coef,pb->exp);
             pb=pb->link;
             break;
         }
     }
     while(pa!=NULL)
     {
         pc=insert(pc,pa->coef,pa->exp);
         pa=pa->link;
     }
     while(pb!=NULL)
     {
         pc=insert(pc,pb->coef,pb->exp);
         pb=pb->link;
     }
     pc->link=NULL;
     pc=ch;
     ch=ch->link;
     free(pc);
     return(ch);
 }
 void print(NODE *head)
 {
     NODE*p=NULL;
     p=head;
     while(p!=NULL)
     {
         printf("%d%d\n",p->coef,p->exp);
         p=p->link;
     }
 }
int main(void)
{
    ah=creat();
    bh=creat();
    ch=polyadd_l(ah,bh);
    print(ch);
}