Сalculator

void solution(){
    stack<char> stk;
    string s;
    cin>>s;
    string up="*/",ni="-+";
    vector<string>res;
    forx(i,0,isz(s)){
        string u="";
        if (isdigit(s[i])){
            while (isdigit(s[i])){
                u+=s[i];
                ++i;
            }
            --i;
            res.push_back(u);
        }
        else if(s[i]=='(')
            stk.push(s[i]);
        else if(s[i]==')'){
            while(stk.top()!='(' && isz(stk)>0){
                u="";
                u+=stk.top();
                res.push_back(u);
                stk.pop();
            }
            if(isz(stk)>0)
                stk.pop();
        }
        else{
            if(isz(stk)==0 || up.find(s[i])!=string::npos && ni.find(stk.top())!=string::npos)
                stk.push(s[i]);
            else{
                while(isz(stk)>0 && (up.find(stk.top())!=string::npos && ni.find(s[i])!=string::npos || up.find(stk.top())!=string::npos && up.find(s[i])!=string::npos  || \
                ni.find(stk.top())!=string::npos && ni.find(s[i])!=string::npos)){
                    u="";
                    u+=stk.top();
                    res.push_back(u);
                    stk.pop();
                }
                stk.push(s[i]);
            }
        }
    }
    string u;
    while(isz(stk)>0){
        u="";
        u+=stk.top();
        res.push_back(u);
        stk.pop();
    }
    stack<int>stk2;
    for(auto& a:res){
        if(up.find(a)==string::npos && ni.find(a)==string::npos)
            stk2.push(stoi(a));
        else{
            int x=stk2.top();
            stk2.pop();
            int y=stk2.top();
            stk2.pop();
            if(a=="+")
                stk2.push(x+y);
            if(a=="-")
                stk2.push(x-y);
            if(a=="*")
                stk2.push(x*y);
            if(a=="/")
                stk2.push(x/y);
        }
    }
    cout<<stk2.top();

}