用单链表表示一元多项式，并实现输入、输出、加、减运算。…… 要求实现： (1)创建多项式 (2)多项式加法 (3)输出多项式 (4)多项式减法 用c语言

在C语言中，我们可以使用结构体和指针来表示一元多项式。首先，我们需要定义一个结构体`Term`，它包含系数（coefficient）和指数（exponent）：

typedef struct {
    int coefficient; // 系数
    int exponent;     // 指数
} Term;

接着，我们定义多项式的结构体`Polynomial`，并包含一个指向`Term`类型的指针数组和长度：

typedef struct Polynomial {
    Term* terms;
    int degree;   // 最大指数
    int size;     // 非零项的数量
} Polynomial;

接下来，分别实现各项功能：

1. **创建多项式** (构造函数)

Polynomial* createPolynomial(int* coefficients, int degree) {
    Polynomial* poly = malloc(sizeof(Polynomial));
    poly->terms = malloc(degree * sizeof(Term));
    for (int i = 0; i < degree; ++i) {
        poly->terms[i].coefficient = coefficients[i];
        poly->terms[i].exponent = i;
        if (coefficients[i] != 0) {
            poly->size++;
        }
    }
    poly->degree = degree;
    return poly;
}

2. **多项式加法**

Polynomial* addPolynomials(Polynomial* a, Polynomial* b) {
    if (a == NULL || b == NULL) return a; // 如果有一个是NULL，返回另一个

    int maxDegree = MAX(a->degree, b->degree);
    Polynomial* result = malloc(sizeof(Polynomial));
    result->terms = malloc(maxDegree * sizeof(Term));
    result->degree = maxDegree;
    result->size = 0;

    for (int i = 0; i <= maxDegree; ++i) {
        Term termA = a->terms[i], termB = b->terms[i];
        result->terms[result->size++] = (termA.coefficient + termB.coefficient) > 0 ?
            {abs(termA.coefficient + termB.coefficient), i} : {0, i};
    }

    free(a->terms);
    free(b->terms);
    return result;
}

3. **输出多项式**

void printPolynomial(Polynomial* poly) {
    for (int i = 0; i <= poly->degree && poly->terms[i].coefficient != 0; ++i) {
        printf("%d*x^%d ", poly->terms[i].coefficient, poly->terms[i].exponent);
    }
    printf("\n");
}

4. **多项式减法**

Polynomial* subtractPolynomials(Polynomial* a, Polynomial* b) {
    if (a == NULL || b == NULL) return a; // 如果有一个是NULL，返回另一个

    Polynomial* result = malloc(sizeof(Polynomial));
    result->terms = malloc(MAX(a->degree, b->degree) * sizeof(Term));
    result->degree = MAX(a->degree, b->degree);
    result->size = 0;

    for (int i = 0; i <= a->degree && i <= b->degree; ++i) {
        Term termA = a->terms[i], termB = b->terms[i];
        result->terms[result->size++] = termA.coefficient - termB.coefficient > 0 ?
            {termA.coefficient - termB.coefficient, i} : {0, i};
    }

    free(a->terms);
    free(b->terms);
    return result;
}

记得处理内存分配失败的情况，并在不需要时释放内存。