point：用两个栈分别存数字和运算符，对于在栈内和在栈外的运算符，需要有两套优先级。

#include <iostream>
#include <string>
#include <stack>
#include <map>

using namespace std;

class Solution {
public:

    int to_postfix(string s)
    {
        int i = 0;

        stack<int> num;
        stack<char> op;
        map<char,int> priority;
        priority['+'] = 0;
        priority['-'] = 0;
        priority['*'] = 1;
        priority['/'] = 1;
        priority['('] = 2;
        priority[')'] = -1;
        map<char,int> in;
        in['('] = -2;
        in[')'] = 3;
        in['+'] = 0;
        in['-'] = 0;
        in['*'] = 1;
        in['/'] = 1;


        while(i<s.size())
        {
            while(i<s.size()&&s[i]==' ') i++;
            if(i==s.size()) break;

            if(s[i]<='9'&&s[i]>='0')
            {
                int temp = 0;
                while(i<s.size()&&s[i]<='9'&&s[i]>='0')
                {
                    temp = temp*10 + s[i]-'0';
                    i++;
                }
                num.push(temp);
                cout<<"num = "<<temp<<endl;
            }
            else
            {
                char curr = s[i];
                cout<<"op is "<<curr<<endl;

                while(!op.empty()&&priority[curr]<=in[op.top()])
                {

                    int num1 = num.top();
                    num.pop();
                    int num2 = num.top();
                    num.pop();
                    int sum;
                    switch(op.top())
                    {
                        case '+':
                            sum = num1+num2;
                            break;
                        case '-':
                            sum = num2-num1;
                            break;
                        case '*':
                            sum = num1*num2;
                            break;
                        case '/':
                            sum = num2/num1;
                            break;
                        default:
                            break;
                    }
                    num.push(sum);
                    cout<<"sum = "<<sum<<endl;
                    op.pop();

                }
                op.push(s[i]);
                i++;
            }
        }
        while(!op.empty()&&num.size()>1)
        {
            if(op.top()==')'||op.top()=='(')
            {
                op.pop();
                continue;
            }
                int num1 = num.top();
                num.pop();
                int num2 = num.top();
                num.pop();
               //cout<<"1. "<<num2<<" "<<op.top()<<"  2."<<num1<<endl;

                int sum;
                switch(op.top())
                    {
                        case '+':
                            sum = num1+num2;
                            break;
                        case '-':
                            sum = num2-num1;
                            break;
                        case '*':
                            sum = num1*num2;
                            break;
                        case '/':
                            sum = num2/num1;
                            break;
                        default:
                            break;
                    }

                op.pop();
                num.push(sum);
        }
        return num.top();

    }
    int calculate(string s) {
       return to_postfix(s);
    }
};
int main()
{

    string str = "32-(3-1)";
    istringstream is(str);
    Solution s1;

    cout<<s1.calculate(str)<<endl;
}