Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

一道混合的题，难度不高，是几个部分组成。

需要注意：
1.转换数字，负数的存在。
2.出栈时数字的顺序。
3.好像没什么了……

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> sk;
        int result;
        for(int i = 0 ; i < tokens.size(); i++)
        {
            if(isop(tokens[i]))
            {
                int second = sk.top();
                sk.pop();
                int first = sk.top();
                sk.pop();
                
                int midresult;
                if(tokens[i] == "+") midresult = first + second;
                else if(tokens[i] == "-")midresult = first - second;
                else if(tokens[i] == "*")midresult = first * second;
                else if(tokens[i] == "/")midresult = first / second;
                
                sk.push(midresult);
                
            }
            else
            {
                int tp = string2num(tokens[i]);
                sk.push(tp);
            }
        }
        return sk.top();
        
    }
    bool isop(string s)
    {
        if(s== "+" ||s== "-" ||s== "*" ||s== "/"  )return 1;
        else return 0;
    }
    int string2num(string s)
    {
        int re =0 ;
        int flag = 0;
        int i = 0;
        if(s[0] == '-')
        {
            flag =1;
            i++;
        }
        for( ; i < s.size();i++)
        {
            re = re*10;
            re = re + s[i]-'0';
        }
        if(flag ==1)
        re = 0 -re;
        return re;
    }
};