leetcode 224: Basic Calculator

My method is a little verbose. The hardest part is how to handle if the '-' is minus or a negative sign.

class Solution {
public:
     int calculate(string s) {
        stack<string> st;
        int len=s.length();
        stringstream ss;
        for(int i=0;i<len;i++)
        {
            if(s[i]==' ')
                continue;
            if(s[i]=='(')
                st.push("(");
            else if(s[i]=='+')
                st.push("+0");
            else if(s[i]=='-')
                st.push("-");
            else if(s[i]>='0'&&s[i]<='9')
            {
                string e;
                if(st.empty()||st.top()=="(")
                    st.push(e+s[i]);
                else
                    st.top().push_back(s[i]);
            }
            else//s[i] is ')', do the calculation until a '('
            {
                int num1=0,num2=0;
                while(1)
                {
                    ss<<st.top();
                    ss>>num1;
                    ss.clear();
                    st.pop();

                    if(st.top()!="(")//() may contain only one number
                    {
                        ss<<st.top();
                        ss>>num2;
                        ss.clear();
                        st.pop();
                    }

                    string temp;
                    ss<<num1+num2;
                    ss>>temp;
                    ss.clear();

                    if(st.top()=="(")
                    {
                        st.pop();
                        if(!st.empty()&&st.top()=="-")
                        {
                            if(temp[0]=='-')
                                st.top()=temp.substr(1);
                            else
                                st.top()+=temp;
                        }
                        else
                            st.push(temp);
                        break;
                    }
                    st.push(temp);
                }
            }
        }
        while(st.size()>1)//no () int the stack any more, finish the rest calculation
        {
            int num1,num2;
            ss<<st.top();
            ss>>num1;
            ss.clear();
            st.pop();

            ss<<st.top();
            ss>>num2;
            ss.clear();

            ss<<num1+num2;
            ss>>st.top();
            ss.clear();
        }
        int res;
        ss<<st.top();
        ss>>res;
        return res;
    }
};