参考答案：

#include<iostream>
#include<stack>
#include<unordered_map>

using namespace std;

unordered_map<char, int> p = { {'+',1},{'-',1},{'*',2},{'/',2} };
stack<char> op;
stack<int> num;

void eval()
{
    int b = num.top();
    num.pop();
    int a = num.top();
    num.pop();
    char c = op.top();
    op.pop();
    int sum = 0;
    if (c == '+') sum = a + b;
    if (c == '-') sum = a - b;
    if (c == '*') sum = a * b;
    if (c == '/') sum = a / b;
    num.push(sum);
}
int main()
{
    string s;
    cin >> s;

    for (int i = 0; i < s.size(); ++i)
    {
        if (isdigit(s[i]))
        {
            int t = 0;
            while (isdigit(s[i]) && i < s.size())
                t = t * 10 + (s[i++] - '0');
            i--;
            num.push(t);
        }
        else if (s[i] == '(') op.push('(');
        else if (s[i] == ')')
        {
            while (op.top() != '(')
                eval();
            op.pop();
        }
        else
        {
            while (op.size() && p[op.top()] >= p[s[i]])
                eval();
            op.push(s[i]);
        }
    }

    while (op.size()) eval();
    cout << num.top() << endl;

    return 0;
}