224. Basic Calculator

class Solution {
public:
    vector<string> ConvertToPostfix(string s){
        vector<string> pp;
        stack<char> op;
        string num = "";
        for(int i=0; i<s.size(); i++){
            if(isdigit(s[i])) num += s[i];
            else{
                if(num.size() > 0) pp.push_back(num);
                num = "";
                if(s[i] == '(') op.push('(');
                else if(s[i] == ')'){
                    while(op.top() != '('){
                        string ss = "";
                        ss += op.top();
                        pp.push_back(ss);
                        op.pop();
                    }
                    op.pop();
                }
                else if(s[i] == '+' || s[i] == '-'){
                    while(!op.empty() && (op.top()=='+' || op.top()=='-')){
                        string ss = "";
                        ss += op.top();
                        pp.push_back(ss);
                        op.pop();
                    }
                    op.push(s[i]);
                }
            }
        }
        if(num.size() > 0) pp.push_back(num);
        while(!op.empty()){
            string ss = "";
            ss += op.top();
            pp.push_back(ss);
            op.pop();
        }
        return pp;
    }

    int calcPostfix(vector<string>& pp){
        stack<int> st;
        for(auto it : pp){
            if(it == "+" || it == "-"){
                int num = st.top();
                st.pop();
                int num2 = st.top();
                st.pop();
                if(it == "+") st.push(num + num2);
                else st.push(num2 - num);
            }
            else st.push(stoi(it));
        }
        return st.top();
    }
    int calculate(string s) {
        string ss = "";
        for(int i=0; i<s.size(); i++){
            if(s[i] != ' ') ss += s[i];
        }
        string sss = "";
        for(int i=0; i<ss.size(); i++){
            if(ss[i] == '-'){
                if(!i || ss[i-1] == '+' || ss[i-1] == '-' || ss[i-1] == '(') sss += '0';
            }
            sss += ss[i];
        }
        auto v = ConvertToPostfix(sss);
        return calcPostfix(v);
    }
};