leetcode_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

计算逆波兰式的值，计算符号只有加减乘除，一看就是需要用到栈模拟，遇到算术符号，就从顶部取数据进行计算，再压入结果。主要注意输入是字符串，需要变成对应数字，一个技巧就是读入到输入输出流中，如下str2num,num2str，十分方便。

class Solution {public:    int evalRPN(vector<string> &tokens) {        return   (int) solve(tokens);    }    long str2num(string s)    {        long num;        stringstream ss(s);        ss>>num;        return num;    }    string num2str(long i)    {        stringstream ss;        ss<<i;        return ss.str();    }    long solve(vector<string> & tokens)    {        stack<string> s;        for(vector<string>::iterator it = tokens.begin();it!=tokens.end();it++)        {           if (*it=="+" || *it=="-" || *it =="*" || *it=="/")           {                long op1,op2,op3;               op2 = str2num(s.top());                s.pop();               op1 = str2num(s.top());               s.pop();               if (*it=="+")                    op3 = op1 + op2;                    else                        if(*it=="-")                            op3 = op1 - op2;                            else                                if(*it=="*")                                    op3 = op1 * op2;                                    else                                        op3 = op1 / op2;                string str=num2str(op3);                s.push(str);           }           else{            s.push(*it);           }        }        return str2num(s.top());    }};