#include<stdio.h>
#include<stdlib.h>
typedef struct Node{
    float coef;
    int exp;
    struct Node *next;
}Node,*Linklist;
Node  *List(){
    struct Node *p,*q;
    struct Node *head;
    head=(Node *)malloc(sizeof(Node));
    if(head == NULL){
    printf("memory error!");
    exit(0);
    }
    head->next=NULL;
    int shuru=0;
    float xishu=0;
    int zhishu=0;
    int i=1;
    p=head;
    printf("请输入多项式n项\n");
    scanf("%d",&shuru);
    for(i=1;i<=shuru;i++){
        q=(Linklist)malloc(sizeof(Node));
        printf("输入第%d项\n",i);
        scanf("%f %d",&xishu,&zhishu);
        p->coef=xishu;
        p->exp=zhishu;
        p->next=q;
        p=q;
    }
    q->next=NULL;
}
Node *change(Linklist p){
    Node *q = p->next;
    float xishu=0;
    int zhishu=0;
    while(q!=NULL){
        if(q->next->exp > q->exp){
            xishu=q->next->coef;q->next->coef=q->coef;q->coef=xishu;
            zhishu=q->next->exp;q->next->exp=q->exp;q->exp=zhishu;
            q=q->next;
        }
        else q=q->next;
    }
}//变为指数为降序
void add(struct Node *La,struct Node *Lb){
    Node *pa=La->next;
    Node *pb=Lb->next;
    Node *Lc=(Node *)malloc(sizeof(Node));
    Node *r=Lc->next;
    Node *m=Lc->next;
    float x;
    int i=0;
    change(La);
    change(Lb);
    while (pa!=NULL && pb!=NULL){
        if(pa->exp > pb->exp){
            r->coef = pa->coef;
            r->exp = pa->exp;
            pa = pa->next;
            r = r->next;
        }
        else if(pa->exp == pb->exp){
            r->coef = pa->coef + pb->coef;
            r->exp = pa->exp;
            pa = pa->next;
            pb = pb->next;
            r = r->next;
        }
        else{
            r->coef = pb->coef;
            r->exp = pb->exp;
            pb = pb->next;
            r = r->next;
        }
    }
    r->next=pa?pa:pb;
    while(m!=NULL){
        i++;
        m = m->next;
    }
    m=Lc->next;
    printf("%d,",i);
    while(m!=NULL){
        if(m->next==NULL){
        printf("%f,%d\n",m->coef,m->exp);
        }
        else{
        printf("%f,%d",m->coef,m->exp);
        }
        m = m->next;
    }
    free(La);
    free(Lb);
}//将两个式子合并并输出
int main(){
    Node *p1 = List();
    Node *p2 = List();
    add(p1,p2);
    return 0;
 }

Node  *List()这个函数里没有返回值。
Node *change(Linklist p)这里的排序是冒泡法，显然是单循环，实现不了排序。
void add(struct Node *La,struct Node *Lb){
    Node *r=Lc->next;            //Lc->next是野指针。
    Node *m=Lc->next;            //同上
   
 while (pa!=NULL && pb!=NULL){        //从这开始，后面开始乱来了
            pa = pa->next;
            r = r->next;        //r->next是什么？？？
        }
两个链表，大多数的情况是链表不会同时结束，那么当这个while运行完之后还要对两个链表进行判断，将没有结束的那个链表接到新的链表上。
free(La);
free(Lb);
链表不是这样释放的，是一个结点一个结点释放。