请用链栈写一个表达式求值的C语言代码

以下是使用链栈实现表达式求值的C语言代码：

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

#define MAX_EXPR_LEN 100 // 最大表达式长度

typedef struct StackNode {
    int data;
    struct StackNode *next;
} StackNode, *LinkStack;

// 创建一个空栈
LinkStack createStack() {
    LinkStack s = (LinkStack) malloc(sizeof(StackNode));
    s->next = NULL;
    return s;
}

// 判断栈是否为空
int isStackEmpty(LinkStack s) {
    return s->next == NULL;
}

// 入栈
void push(LinkStack s, int data) {
    StackNode *node = (StackNode*) malloc(sizeof(StackNode));
    node->data = data;
    node->next = s->next;
    s->next = node;
}

// 出栈
int pop(LinkStack s) {
    if (isStackEmpty(s)) {
        printf("stack is empty!");
        exit(1);
    }
    StackNode *top = s->next;
    int data = top->data;
    s->next = top->next;
    free(top);
    return data;
}

// 获取栈顶元素
int getTop(LinkStack s) {
    if (isStackEmpty(s)) {
        printf("stack is empty!");
        exit(1);
    }
    return s->next->data;
}

// 表达式求值
int eval(char expr[]) {
    LinkStack numStack = createStack(); // 数字栈
    LinkStack opStack = createStack();  // 操作符栈

    int i = 0;
    while (expr[i] != '\0') {
        if (isdigit(expr[i])) { // 数字直接入栈
            int num = 0;
            while (isdigit(expr[i])) {
                num = num * 10 + (expr[i] - '0');
                i++;
            }
            push(numStack, num);
        } else if (expr[i] == '+' || expr[i] == '-') {
            while (!isStackEmpty(opStack) && (getTop(opStack) == '+' || getTop(opStack) == '-' || getTop(opStack) == '*' || getTop(opStack) == '/')) {
                int b = pop(numStack);
                int a = pop(numStack);
                char op = pop(opStack);
                int result;
                if (op == '+') {
                    result = a + b;
                } else if (op == '-') {
                    result = a - b;
                } else if (op == '*') {
                    result = a * b;
                } else {
                    result = a / b;
                }
                push(numStack, result);
            }
            push(opStack, expr[i]);
            i++;
        } else if (expr[i] == '*' || expr[i] == '/') {
            while (!isStackEmpty(opStack) && (getTop(opStack) == '*' || getTop(opStack) == '/')) {
                int b = pop(numStack);
                int a = pop(numStack);
                char op = pop(opStack);
                int result;
                if (op == '*') {
                    result = a * b;
                } else {
                    result = a / b;
                }
                push(numStack, result);
            }
            push(opStack, expr[i]);
            i++;
        } else if (expr[i] == '(') {
            push(opStack, expr[i]);
            i++;
        } else if (expr[i] == ')') {
            while (getTop(opStack) != '(') {
                int b = pop(numStack);
                int a = pop(numStack);
                char op = pop(opStack);
                int result;
                if (op == '+') {
                    result = a + b;
                } else if (op == '-') {
                    result = a - b;
                } else if (op == '*') {
                    result = a * b;
                } else {
                    result = a / b;
                }
                push(numStack, result);
            }
            pop(opStack); // 弹出左括号
            i++;
        } else {
            printf("invalid expr!");
            exit(1);
        }
    }

    while (!isStackEmpty(opStack)) {
        int b = pop(numStack);
        int a = pop(numStack);
        char op = pop(opStack);
        int result;
        if (op == '+') {
            result = a + b;
        } else if (op == '-') {
            result = a - b;
        } else if (op == '*') {
            result = a * b;
        } else {
            result = a / b;
        }
        push(numStack, result);
    }

    int result = pop(numStack);
    return result;
}

int main() {
    char expr[MAX_EXPR_LEN];
    printf("请输入表达式：");
    scanf("%s", expr);
    int result = eval(expr);
    printf("表达式的值为：%d\n", result);
    return 0;
}

以上代码实现了一个支持加、减、乘、除、括号的表达式求值的功能。