一元稀疏多项式求导数的函数在main之前已经声明了，但是仍然说未找到标识符，求解惑

 #include<stdio.h>
#include<stdlib.h>
typedef struct PolyNode *Polynomial;
struct PolyNode {
    int coef;//系数
    int expon;//指数
    Polynomial link;//后继指针
};

void display();//交互界面
void testIO();//多项式输入输出测试
void calculate();//多项式计算
void Attach(int c, int e, Polynomial *pRear);//向多项式链表中插入结点
Polynomial ReadPoly();//读取多项式链表
Polynomial Add(Polynomial P1, Polynomial P2);//多项式相加
Polynomial daoshu(Polynomial P1, Polynomial P2);//导数
Polynomial Mult(Polynomial P1, Polynomial P2);//多项式相乘
Polynomial Combin(Polynomial P);//合并同类项
Polynomial Rev(Polynomial P);//多项式每一项系数取相反数
void PrintDown(Polynomial P, int flag);//降幂输出

int main()
{   int option;
    while (1) 
    {   system("cls");    display();
        scanf("%d", &option);    getchar(); fflush(stdin);
        switch (option) {
        case 1:        testIO();        break;//多项式输入输出测试
        case 2:        calculate();    break;//多项式计算
        case 3:        return 0;//退出程序
        default:    printf("输入有误，请重新输入\n");
            system("pause");    break;
        }
    }
}
//交互界面
void display() {
    printf("**************一元多项式计算器**************\n");
    printf("1.多项式输入输出测试\n");
    printf("2.多项式计算\n");
    printf("3.退出程序\n");
    printf("请输入您的选择：");
}



//导数
Polynomial daoshu(Polynomial P1, Polynomial P2)
{   Polynomial q=p1->link,l,g,h;
    g=l= (Polynomial)malloc(sizeof(struct PolyNode));
    g->link=NULL;
    while(q)
    {if(q->expon!=0)
    {h=(Polynomial)malloc(sizeof(struct PolyNode));
    h->coef=q->coef*q->expon;
    h->expon=q->expon-1;
    h->link=l->link;
    l->link=h;
    l=h;
    }
    return(g)
}

//多项式输入输出测试
void testIO() {
    printf("多项式输入输出测试:\n");
    Polynomial L = ReadPoly();//读入多项式
    PrintDown(L, 0);//降序输出
    printf("测试完毕\n");    system("pause");
}
//多项式计算
void calculate() {
    Polynomial P1, P2, Res;
    char flag;    int option;
    printf("多项式计算:\n");
    while (1) {
        printf("请输入运算符号(仅支持+、-、*和导数（导数请输入！）,输入#返回上一层)：");
        scanf("%c", &flag);    getchar();    fflush(stdin);
        switch (flag) {
        case '+':    option = 1;    break;
        case '-':    option = 2;    break;
        case '*':    option = 3;    break;
        case '！':    option = 4;    break;
        case '#':    return;
        default:    printf("输入有误，请重新输入\n");
            system("pause");    option = -1; break;
        }
        if (option != -1) {
            P1 = ReadPoly();    getchar();    fflush(stdin);
            P2 = ReadPoly();    getchar();    fflush(stdin);
            if (option == 1) { Res = Add(P1, P2);}
            else if (option == 2) { Res = Add(P1, Rev(P2)); }
            else if (option == 3) { Res = Mult(P1, P2); }
            else if (option == 4) { Res = daoshu(P1,P2); }
            PrintDown(Res, 0);    system("pause");
            
        }
    }
}

//向多项式链表中插入结点（尾插法）
void Attach(int c, int e, Polynomial *pRear) {
    Polynomial P;
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    if (c == 0) { return; }
    else { P->coef = c;    P->expon = e; }
    P->link = NULL;    (*pRear)->link = P;        *pRear = P;
}
//读取多项式链表
Polynomial ReadPoly() {
    Polynomial P, Rear, t;
    int n, c, e;
    printf("输入多项式的项数：");
    scanf("%d", &n);
    if (n <= 0) { printf("输入有误\n"); return NULL; }
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    P->link = NULL;    Rear = P;
    printf("输入多项式的系数与指数：\n");
    while (n--) {
        scanf("%d%d", &c, &e);
        Attach(c, e, &Rear);
    }
    t = P; P = P->link; free(t);
    P = Combin(P);
    return P;
}
//多项式相加
Polynomial Add(Polynomial P1, Polynomial P2)
{
    Polynomial P, Rear, t;
    Polynomial t1 = P1;
    Polynomial t2 = P2;
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    P->link = NULL;    Rear = P;
    while (t1&&t2)
    {
        if (t1->expon == t2->expon)
        {
            if (t1->coef + t2->coef)
            {
                Attach(t1->coef + t2->coef, t1->expon, &Rear);
                t1 = t1->link;    t2 = t2->link;
            }
        }
        else if (t1->expon > t2->expon)
        {
            Attach(t1->coef, t1->expon, &Rear);
            t1 = t1->link;
        }
        else
        {
            Attach(t2->coef, t2->expon, &Rear);
            t2 = t2->link;
        }
    }
    while (t1)
    {
        Attach(t1->coef, t1->expon, &Rear);
        t1 = t1->link;
    }
    while (t2)
    {
        Attach(t2->coef, t2->expon, &Rear);
        t2 = t2->link;
    }
    Rear->link = NULL;
    t = P;    P = P->link;    free(t);
    P = Combin(P);
    return P;
}
//多项式相乘
Polynomial Mult(Polynomial P1, Polynomial P2) {
    Polynomial P, Rear, t1, t2, t;
    int c, e;
    if (!P1 || !P2)return NULL;
    t1 = P1; t2 = P2;
    P = (Polynomial)malloc(sizeof(struct PolyNode));
    P->link = NULL;    Rear = P;
    while (t2) {
        Attach(t1->coef*t2->coef, t1->expon + t2->expon, &Rear);
        t2 = t2->link;
    }
    t1 = t1->link;
    while (t1) {
        t2 = P2; Rear = P;
        while (t2) {
            e = t1->expon + t2->expon;
            c = t1->coef*t2->coef;
            while (Rear->link&&Rear->link->expon > e) {
                Rear = Rear->link;
            }
            if (Rear->link&&Rear->link->expon == e) {
                if (Rear->link->coef + c) {
                    Rear->link->coef += c;
                }
                else {
                    t = Rear->link;
                    Rear->link = t->link;
                    free(t);
                }
            }
            else {
                t = (Polynomial)malloc(sizeof(struct PolyNode));
                t->coef = c; t->expon = e;
                t->link = Rear->link;
                Rear->link = t; Rear = Rear->link;
            }
            t2 = t2->link;
        }
        t1 = t1->link;
    }
    t2 = P; P = P->link; free(t2);
    P = Combin(P);
    return P;
}
//合并同类项
Polynomial Combin(Polynomial L) {
    Polynomial Head = L;
    Polynomial P;
    Polynomial Pr = L;
    if (!L) {
        P = (Polynomial)malloc(sizeof(struct PolyNode));
        P->coef = 0;    P->expon = 0;    P->link = NULL;
        return L;
    }
    P = L->link;
    while (Head) {
        while (P) {
            if (P->expon == Head->expon) {
                Head->coef = Head->coef + P->coef;
                if (Head->coef == 0) { Head->expon = 0; }
                Pr->link = P->link; free(P); P = Pr->link;
            }
            else {
                Pr = Pr->link;    P = Pr->link;
            }
        }
        Head = Head->link;    Pr = Head;
        if (Head)P = Head->link;
    }
    //L = Up(L);
    return L;
}
//多项式每一项系数取相反数
Polynomial Rev(Polynomial P) {
    Polynomial node = P;
    if (!P) { return NULL; }
    while (node) {
        node->coef = -(node->coef);
        node = node->link;
    }
    return P;
}

//降幂输出
void PrintDown(Polynomial P, int flag) {
    if (flag == 0) { printf("结果如下(降幂)："); }
    if (!P) { printf("0\n"); return; }
    if (P->link) { PrintDown(P->link, 1); }
    if (P->coef >= 0 && P->link != NULL) { printf("+"); }
    if (P->coef == 0 || P->expon == 0) { printf("%d", P->coef); }
    else { printf("%dx^%d", P->coef, P->expon); }
    if (flag == 0) { printf("\n"); }
}