一元多项式的乘法 合并同类项

#include<stdio.h>
#include<malloc.h>
typedef int elementtype;
struct node{
    elementtype dishu,zhishu;
    struct node* next;
};
typedef struct node* List;
List create(List L);
List add(List L1,List L2);
List mult(List L1,List L2);
int main(){
    List L1=NULL,L2=NULL;
    L1=create(L1);
    L2=create(L2);
    List p=mult(L1,L2);
    int flag=1;
    while(p){
        if(flag==1){
            printf("%d %d",p->dishu ,p->zhishu );
            flag=0;
        }
        else printf(" %d %d",p->dishu ,p->zhishu );
        p=p->next ;////
    } 
    flag=1;
    printf("\n");
    List head=add(L1,L2); 
    while(head){
        if(flag==1){
        printf("%d %d",head->dishu ,head->zhishu );
        flag=0;    
    }
        else printf(" %d %d",head->dishu ,head->zhishu );
        head=head->next ;
    }
    return 0;
}
List create(List L){
    elementtype number;
    List p,tail=NULL;
    int n,i;
    scanf("%d",&n);
    for(i=0;i<n;i++){
        p=(List)malloc(sizeof(struct node));
        p->next =NULL;
        scanf("%d %d",&p->dishu,&p->zhishu );
        if(L==NULL)L=p;
        else{
            tail->next =p;
        }
        tail=p;
    }
    return L;
}
List add(List L1,List L2){
    List p1=L1,p2=L2;
    List head=NULL,tail,p;
    while(1){
        if(p1==NULL||p2==NULL)break;
        p=(List)malloc(sizeof(struct node));
        p->next =NULL;
        if(p1->zhishu ==p2->zhishu ){
            p->dishu =p1->dishu +p2->dishu ;
            p->zhishu =p1->zhishu ;
            p1=p1->next ;
            p2=p2->next ;//
        }
        else if(p1->zhishu >p2->zhishu ){
            p->dishu =p1->dishu ;
            p->zhishu =p1->zhishu;
            p1=p1->next ; //
        }
        else{
            p->dishu =p2->dishu ;
            p->zhishu =p2->zhishu; 
            p2=p2->next ;//
        }
        if(head==NULL)head=p;
        else tail->next =p;
        tail=p;
    }
    if(p1==NULL) tail->next =p2;
    else tail->next =p1;
    return head;
}
List mult(List L1,List L2){
    List head=NULL,p,tail;
    List p1=L1,p2=L2;
    while(p2){//初步结果 
        p=(List)malloc(sizeof(struct node));
        p->next =NULL;
        p->dishu =p1->dishu *p2->dishu ;
        p->zhishu =p1->zhishu+p2->zhishu;
        p2=p2->next ;  
        if(head==NULL)head=p;
        else tail->next =p;
        tail=p;
    }
    p1=p1->next ;
    while(p1){
            int equal;
            p2=L2;////
            while(p2){
            List rear=head;
            equal=0;
            p=(List)malloc(sizeof(struct node));
            p->next =NULL;
            p->dishu =p1->dishu *p2->dishu ;
            p->zhishu =p1->zhishu+p2->zhishu;
            while(rear->next&&rear->next->zhishu >=p->zhishu ){
            if(rear->next ->zhishu ==p->zhishu ){//相等 
                rear->next->dishu =p->dishu +rear->next ->dishu ;//
                if(rear->next->dishu==0) rear->next =rear->next->next ; ////
                equal=1;
                break; 
            }
            rear=rear->next ;
        }
            if(equal==0){//不相等 
            p->next =rear->next ;
            rear->next =p;
        }
        p2=p2->next ;
        }
        p1=p1->next ;
    }
    return head;
}

合并同类项的语句哪里出错了？出错点在哪？

大哥我能问问这是啥做题软件吗