经典。

#include<bits/stdc++.h>
using namespace std;

stack<char> ops;
stack<int> nums;

bool is_num(char c)
{
    if(c >= '0' && c <= '9') return true;
    return false;
}

void cal()
{
    int b = nums.top(); nums.pop();
    int a = nums.top(); nums.pop();
    char c = ops.top(); ops.pop();
    int res;

    if(c == '+') res = a + b;
    else if(c == '-') res = a - b;
    else if(c == '*') res = a * b;
    else if(c == '/') res = a / b;

    nums.push(res);
}


int main()
{
    string s;
    cin >> s; s = '(' + s + ')';

    for(int i = 0; i < s.size(); i++)
    {
        char c = s[i];

        if(is_num(c))
        {
            int j = i, num = 0;
            while(is_num(s[j])) num = num * 10 + (s[j] - '0'), j++;
            i = j - 1; nums.push(num);
        }
        else
        {
            if(c == '(') ops.push(c);
            else if(c == '+' || c == '-')
            {
                while(ops.top() != '(') cal();
                ops.push(c);
            }
            else if(c == '*' || c == '/')
            {
                while(ops.top() == '*' || ops.top() == '/') cal();
                ops.push(c);
            }
            else if(c == ')')
            {
                while(ops.top() != '(') cal();
                ops.pop();
            }
            else cout << "NO" << endl;
        }
    }

    cout << nums.top() << endl;

    return 0;
}