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

计算逆波兰表达式

class Solution {public:    int calc(int & l,int & r,const string & oper){        if(oper=="+") return l+r;        if(oper=="-") return l-r;        if(oper=="*") return l*r;        if(oper=="/") return l/r;        throw "wrong oper";    }    int eval(vector<string>::const_reverse_iterator & ita){        const string & oper = *ita;        if(oper == "+" || oper == "-" || oper == "*" || oper == "/"){ //运算符号            ++ita;            int r = eval(ita);            int l= eval(ita);            return calc(l,r,oper);        }else{            ++ita;            return atoi(oper.c_str());        }    }    int evalRPN(vector<string> &tokens) {        vector<string>::const_reverse_iterator  ita = tokens.rbegin();        return eval(ita);    }};