表达式中数字均为非负整数

stack<int> num;
stack<char> op;

void eval()
{
    int b = num.top();
    num.pop();
    int a = num.top();
    num.pop();
    char c = op.top();
    op.pop();
    if (c == '+')
    {
        num.push(a + b);
    }
    else if (c == '-')
    {
        num.push(a - b);
    }
    else if (c == '*')
    {
        num.push(a * b);
    }
    else
    {
        num.push(a / b);
    }
}

int main()
{
    std::ios::sync_with_stdio(false), std::cin.tie(0), std::cout.tie(0);
    unordered_map<char, int> pr = {{'+', 1}, {'-', 1}, {'*', 2}, {'/', 2}};//运算符优先级
    string str;
    cin >> str;
    for (int i = 0; i < str.length(); i++)
    {
        if (str[i] >= '0' && str[i] <= '9')
        {
            int x = 0;
            while (str[i] >= '0' && str[i] <= '9')
            {
                x = 10 * x + str[i++] - '0';
            }
            num.push(x);
            i--;
        }
        else if (str[i] == ')')
        {
            while (op.top() != '(')
            {
                eval();
            }
            op.pop();
        }
        else
        {
            while (op.size() && str[i] != '(' && pr[str[i]] <= pr[op.top()])
            {
                eval();
            }
            op.push(str[i]);
        }
    }
    while (op.size())
    {
        eval();
    }
    cout << num.top() << endl;
    return 0;
}