能够按照指数降序排列建立并输出多项式；能够完成两个多项式的输出、 相加、相 减，并将结果输出。 【任务】 将任意给出的 n 次多项式建成单链表结构，结点的结构为系数、指数、指针，然后 实现多项式相关运算。 良好的用户界面，对于错误的输入，要能给出信息提示。

为了实现这个功能，你需要定义一个多项式结点的数据结构，包含系数、指数以及指向下一个结点的指针。同时，你需要创建函数用于构建、操作多项式链表以及处理用户输入。

首先，定义一个多项式结点的数据结构：

typedef struct PolynomialNode {
    double coefficient;  // 系数
    int exponent;       // 指数
    struct PolynomialNode* next;  // 指向下一个结点的指针
} Polynomial;

// 构建空多项式链表
Polynomial* buildEmptyPolynomial() {
    Polynomial* head = malloc(sizeof(Polynomial));
    head->coefficient = 0;
    head->exponent = -1;
    head->next = NULL;
    return head;
}

// 根据给定的系数和指数添加结点到链表
void addTermToPolynomial(Polynomial** polynomial, double coefficient, int exponent) {
    Polynomial* newNode = malloc(sizeof(Polynomial));
    newNode->coefficient = coefficient;
    newNode->exponent = exponent;
    newNode->next = *polynomial;
    *polynomial = newNode;
}

// 对多项式进行按指数降序排序
void sortPolynomialByExponentDesc(Polynomial** polynomial) {
    if (*polynomial == NULL) return;
    qsort(*polynomial, 1, sizeof(Polynomial), compareExponents);
}

// 比较指数函数
int compareExponents(const void* a, const void* b) {
    const Polynomial* nodeA = (const Polynomial*)a;
    const Polynomial* nodeB = (const Polynomial*)b;
    return nodeB->exponent - nodeA->exponent;
}

// 输出多项式
void printPolynomial(const Polynomial* polynomial) {
    Polynomial* current = polynomial;
    printf("( ");
    while (current != NULL) {
        printf("%.2fX^%d ", current->coefficient, current->exponent);
        current = current->next;
    }
    printf(")");
}

// 多项式加法和减法
Polynomial* addOrSubtractPolynomials(Polynomial* polynomial1, Polynomial* polynomial2, char operation) {
    Polynomial* result = buildEmptyPolynomial();
    Polynomial* current1 = polynomial1, *current2 = polynomial2;
    
    while (current1 && current2) {
        if (current1->exponent > current2->exponent) {
            addTermToPolynomial(&result, current1->coefficient, current1->exponent);
            current1 = current1->next;
        } else if (current1->exponent < current2->exponent) {
            addTermToPolynomial(&result, current2->coefficient, current2->exponent);
            current2 = current2->next;
        } else {
            switch(operation) {
                case '+':
                    result->coefficient += current1->coefficient;
                    break;
                case '-':
                    result->coefficient -= current1->coefficient;
                    break;
                default:
                    printf("Invalid operation. Please use '+' or '-' for addition or subtraction.\n");
                    return result;
            }
            current1 = current1->next;
            current2 = current2->next;
        }
    }

    while (current1) {
        addTermToPolynomial(&result, current1->coefficient, current1->exponent);
        current1 = current1->next;
    }

    while (current2) {
        addTermToPolynomial(&result, current2->coefficient, current2->exponent);
        current2 = current2->next;
    }

    return result;
}

// 用户输入处理函数
void handleUserInput() {
    Polynomial* polynomial1 = buildEmptyPolynomial(), *polynomial2;
    char operation;
    printf("Enter the coefficients and exponents of the first polynomial separated by 'x': ");
    scanf("%lf x %lf", &polynomial1->coefficient, &polynomial1->exponent);

    while (scanf(" x %lf", &polynomial1->next->exponent) == 1) {
        addTermToPolynomial(&polynomial1, 1, polynomial1->next->exponent);
        scanf("%lf", &polynomial1->next->coefficient); // 可能需要验证非零输入
        polynomial1->next = polynomial1->next->next;
    }

    printf("Enter operation (+/-): ");
    scanf(" %c", &operation);

    Polynomial* result;
    if (operation == '+') {
        scanf("Enter the coefficients and exponents of the second polynomial separated by 'x': ");
        // 重复上述过程处理第二个多项式
        polynomial2 = ...; // 处理第二个多项式的输入
        result = addOrSubtractPolynomials(polynomial1, polynomial2, operation);
    } else {
        result = addOrSubtractPolynomials(polynomial1, NULL, operation); // 减去空多项式，相当于原地修改
    }
    printPolynomial(result);
}

这部分代码实现了多项式链表的构建、排序、输出，以及加法和减法运算。用户通过交互输入两个多项式及其对应的操作。注意，处理用户输入的部分未完全写出，你需要在`handleUserInput`函数内完善第二个多项式的输入部分。