我这个 一元多项式相加 结果的系数为什么不变呢？（万分感谢）

#include "stdlib.h" 

#include <stdio.h> 

typedef struct LNode  

{  

        int coef;  

        int expn;  

        struct LNode *next;  

}LNode,*ListNode;  



ListNode CreateFromTail()  

{  

         ListNode l,p,tail;  

         int m;  

         int n;  

         l=(ListNode)malloc(sizeof(LNode));  

         l->next=NULL;  

         tail=l;  

         printf("\nplease put the coef:");  

         scanf("%d",&m);  

         printf("\nplease put the expn:");  

         scanf("%d",&n);  

         while(m)  

         {  

            p=(ListNode)malloc(sizeof(LNode));  

            p->coef=m;  

            p->expn=n;  

            tail->next=p;  

            tail=p;  

            printf("\nplease put the coef:");  

            scanf("%d",&m);  

            printf("\nplease put the expn:");  

            scanf("%d",&n);  

         }  

         tail->next=NULL;  

         return l;  

}  

ListNode MergeList(ListNode la,ListNode lb)  

{  

         ListNode ra,rb,pa,pb;  

         int sum;  

         ra=la;  

         rb=lb;  

         pa=la->next;  

         pb=lb->next;  //这里你原先是 pb=la->next;

         while(pa&&pb)  

         {  

              if(pa->expn>pb->expn)  

              {  

                ra=ra->next;  

                pa=pa->next;  

              }  

              else if(pa->expn<pb->expn)  

              {  

                 rb->next=pb->next;  

                 pb->next=ra->next;  

                 ra->next=pb;  

                 ra=ra->next;  

                 pa=ra->next;  

                 pb=rb->next;  

              }  

              else  

              {  

                 if((pa->coef+pb->coef)!=0)  

                 {  

                    sum=pa->coef+pb->coef;  

                    pa->coef=sum;  

                    ra=ra->next;  

                    pa=pa->next;  

                    rb=rb->next;  

                    pb=pb->next;  

              }  

              else  

              {  

                 pa=pa->next;  

                 pb=pb->next;  

                 ra->next=pa;  

                 rb->next=pb;  

              }  

         }  

       }  

       if(pb)  

       ra->next=pb;  

       free(lb);  

       return la;  

}  

main()  

{  

   ListNode m,n,l,p;  

   printf("the first equation:");  

   m=CreateFromTail();  

    printf("\n");  

    printf("the second equation:");  

    n=CreateFromTail();  

    l=MergeList(m,n);  

    p=l->next;  

    while(p!=NULL)  

    {  

       printf(" %d %d\t",p->coef,p->expn);  

       p=p->next;  

    }  

    system("pause");

}