CTDL&TT: Tính toán hậu tố

//Hau to
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <conio.h>

int stack [1000];
int top;

// Tính giá tr. c.a bi.u th.c
int eval (char *s);
// Ki.m tra có ph.i phép toán
int isop (char op);
// Thao tác ð.y ra c.a ngãn x.p
int pop (void);
// Thao tác ð.y vào c.a ngãn x.p
void push (int a);
// Th.c hi.n phép toán
int do_op (int a, int b, char op);

int main (void){
    char expression[80];
    int value;

    printf ("Nhap vao xau bieu thuc: ");
    gets(expression);
    printf ("\nBieu thuc nhap vao: %s", expression);
    value=eval (expression);
    printf ("\nGia tri cua bieu thuc = %i", value);
    getch();

    return 0;
}

int eval (char *s){
    char *ptr;
    int first, second, c;
    ptr = strtok (s, " ");
    top = -1;
    while (ptr) {
        if (isop (*ptr)) {
        second = pop(); first = pop();
        c = do_op (first, second, *ptr);
        push(c);
        }
        else { c = atoi(ptr); push(c);
        }
    ptr = strtok (NULL, " ");
    }
    return (pop ());
}

int do_op (int a, int b, char op){
    int ans;

    switch (op) {
        case '+':
        ans = a + b;
        break;
        case '-':
        ans = a - b;
        break;
        case '*':
        ans = a * b;
    break;
    }

    return ans;
}

int pop (void){
    int ret;
    ret = stack [top];
    top--;
    return ret;
}

void push (int a){
    top++;
    stack [top] = a;
}

int isop (char op){
    if (op == '+' || op == '-' || op == '*')
        return 1;
    else
        return 0;
}