Node * Add(Node *a, Node *b)
{
    Node *p, *q, *pre,*head;
    head=(Node*)malloc(sizeof(Node));
    p = a; q = b;
    pre = head;
    pre->next=pre;
    while (p != NULL&&q != NULL)
    {
        if (p->n < q->n)
        {
            pre=pre->next;
            pre->next = (Node*)malloc(sizeof(Node));
            pre->a=q->a ;
            pre->n=q->n;
            q=q->next;
        }
        else if (p->n == q->n)
        {
            pre=pre->next;
            pre->next = (Node*)malloc(sizeof(Node));
            pre->a=p->a + q->a ;
            pre->n=q->n;
            q=q->next;
            p=p->next;
        }
        else
        {
            pre=pre->next;
            pre->next = (Node*)malloc(sizeof(Node));
            pre->a= p->a ;
            pre->n=p->n;
            p=p->next;
        }
    }
      while(p||q)
    {
        if (p != NULL)
        {
            pre=pre->next;
            pre->next = (Node*)malloc(sizeof(Node));
            pre->a=p->a;
            pre->n=p->n;
            p=p->next;
        }

        else
        {
            pre=pre->next;
            pre->next = (Node*)malloc(sizeof(Node));
            pre->a=q->a;
            pre->n=q->n;
            q=q->next;
        }
    }
    pre->next=NULL;
    return head;

楼主，在main函数里调用自定义函数 ,CTRL+F5调试运行。