要求:用c语言实现代码，将一元多项式采用单链表存储结构，实现如下功能: 1)从键盘输人一元多项式的各项系数和指数，创建一 元多项式链表;2)输出一元多项式; 3)任意输入的两个多项式相加，求和多项式. 4)任意输人的两个多项式相减，求差多项式。

在C语言中，我们可以使用结构体表示多项式项，并使用链表的数据结构来存储多项式。这里是一个简单的实现，包括了创建、显示和操作多项式的功能：

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    double coef; // 系数
    int exp;   // 指数
} PolynomialTerm;

typedef struct {
    PolynomialTerm* terms;
    int count;
} Polynomial;

// 创建新的多项式项
PolynomialTerm* new_term(double coef, int exp) {
    PolynomialTerm* term = (PolynomialTerm*)malloc(sizeof(PolynomialTerm));
    term->coef = coef;
    term->exp = exp;
    return term;
}

// 添加新项到多项式
void add_term(Polynomial* poly, double coef, int exp) {
    PolynomialTerm* new = new_term(coef, exp);
    if (poly->count == 0) {
        poly->terms = new;
    } else {
        poly->terms[poly->count - 1].next = new;
    }
    poly->count++;
}

// 显示多项式
void display_polynomial(Polynomial poly) {
    for (int i = poly.count - 1; i >= 0; i--) {
        printf("%f * x^%d + ", poly.terms[i].coef, poly.terms[i].exp);
    }
    printf("0\n");
}

// 多项式相加
Polynomial add_poly(Polynomial poly1, Polynomial poly2) {
    Polynomial result = {NULL, 0};
    for (int i = 0; i < poly1.count || i < poly2.count; i++) {
        if (i < poly1.count && i < poly2.count) {
            PolynomialTerm* sum_term = new_term(poly1.terms[i].coef + poly2.terms[i].coef, poly1.terms[i].exp);
        } else if (i < poly1.count) {
            PolynomialTerm* term = poly1.terms[i];
        } else {
            PolynomialTerm* term = poly2.terms[i - poly1.count];
        }
        result.count++;
        result.terms = realloc(result.terms, sizeof(PolynomialTerm*) * (result.count + 1));
        result.terms[result.count - 1] = term;
    }
    return result;
}

// 主函数
int main() {
    Polynomial poly1, poly2;
    while (true) {
        char choice;
        printf("1. 输入多项式并添加\n2. 显示多项式\n3. 加法多项式\n4. 减法多项式\n5. 退出\n");
        scanf(" %c", &choice);
        switch (choice) {
            case '1':
                double coef, exp;
                printf("请输入系数和指数，以空格分隔：");
                scanf("%lf %d", &coef, &exp);
                add_term(&poly1, coef, exp);
                break;
            case '2':
                display_polynomial(poly1);
                break;
            case '3':
                if (poly1.count > 0 && poly2.count > 0) {
                    Polynomial result = add_poly(poly1, poly2);
                    display_polynomial(result);
                } else {
                    printf("请输入两个非空多项式进行加法操作。\n");
                }
                break;
            case '4':
                if (poly1.count > 0 && poly2.count > 0) {
                    Polynomial result = {NULL, 0};
                    for (int i = 0; i < poly1.count; i++) {
                        PolynomialTerm* diff_term = new_term(poly1.terms[i].coef - poly2.terms[i].coef, poly1.terms[i].exp);
                        result.count++;
                        result.terms = realloc(result.terms, sizeof(PolynomialTerm*) * (result.count + 1));
                        result.terms[result.count - 1] = diff_term;
                    }
                    display_polynomial(result);
                } else {
                    printf("请输入两个非空多项式进行减法操作。\n");
                }
                break;
            case '5':
                exit(0);
            default:
                printf("无效选择，请再试一次。\n");
        }
    }
    return 0;
}

这个程序会提示用户选择操作，输入多项式，显示多项式，以及对两个多项式进行加法和减法。注意，为了简化示例，我们假设用户总是输入有效的数据。实际应用中应增加错误处理。