利用栈进行表达式求值

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

#define ERROR 0
#define OK 1

typedef struct Stack {
    int *elements;
    int max_size, top_index;
} Stack;

void init(Stack *s, int length) {
    s->elements = (int *)malloc(sizeof(int) * length);
    s->max_size = length;
    s->top_index = -1;
}

int push(Stack *s, int element) {
    if (s->top_index >= s->max_size - 1) {
        return ERROR;
    }
    s->top_index++;
    s->elements[s->top_index] = element;
    return OK;
}

int pop(Stack *s) {
    if (s->top_index < 0) {
        return ERROR;
    }
    s->top_index--;
    return OK;
}

int top(Stack *s) {
    return s->elements[s->top_index];
}

int empty(Stack *s) {
    if (s->top_index < 0) {
        return 1;
    } else {
        return 0;
    }
}

int precede(char a, char b) {
    if (a == '*' && b == '+') {
        return 1;
    } else {
        return 0;
    }
}

int operate(char theta, int a, int b) {
    if (theta == '+') {
        return a + b;
    } else {
        return a * b;
    }
}

void calc(Stack *numbers, Stack *operators) {
    int a = top(numbers);
    pop(numbers);
    int b = top(numbers);
    pop(numbers);
    push(numbers,operate(top(operators),a,b));
    pop(operators);
}

void clear(Stack *s) {
    free(s->elements);
    free(s);
}

int main() {
    int n;
    scanf("%d", &n);
    Stack *numbers = (Stack *)malloc(sizeof(Stack));
    init(numbers, n);
    Stack *operators = (Stack *)malloc(sizeof(Stack));
    init(operators, n);
    char *buffer = (char *)malloc(sizeof(char) * (n + 1));
    scanf("%s", buffer);
    int i = 0;
    while (i < n) {
        if (isdigit(buffer[i])) {
            push(numbers, buffer[i] - '0');
            i++;
        }else{
            if (empty(operators) || precede(buffer[i], top(operators))){
                push(operators,buffer[i]);
                i++;
            }
            else{
                calc(numbers,operators);
            }
        }
    }
    while (!empty(operators)) {
        calc(numbers,operators);
    }
    printf("%d\n",top(numbers));
    clear(numbers);
    clear(operators);
    free(buffer);
    return 0;
}