Evaluate 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

int stoi(string str)

    {

        bool flag = true;

        if (str[0] == '-') flag = false;

        int ret = 0, i;

        i = (flag) ? 0 : 1;

        for (; i<str.size(); i++)

        {

            ret = ret * 10 + (str[i] - '0');

        }

        return (flag) ? ret : (-ret);

    }



    int evalRPN(vector<string> &tokens)

    {

        int ans = 0;

        stack<int> s;

        int a, b, c;

        for (int i = 0; i < tokens.size(); i++)

        {

            if (tokens[i] != "+"&&tokens[i] != "-"&&tokens[i] != "*"&&tokens[i] != "/")

            {

                string str = tokens[i];

                int temp = stoi(str);

                s.push(temp);

            }

            else

            {

                a = s.top();

                s.pop();

                b = s.top();

                s.pop();

                if (tokens[i] == "+") c = b + a;

                if (tokens[i] == "-") c = b - a;

                if (tokens[i] == "*") c = b*a;

                if (tokens[i] == "/") c = b / a;

                s.push(c);

            }

        }

        ans = s.top();

        return ans;

    };

int _tmain(int argc, _TCHAR* argv[])

{

    Solution *sln = new Solution();

    //string s = "  hello dd  ";

    //sln->reverseWords(s);

    //cout << s << endl;



    vector<string> svec;

    svec.push_back("4");

    svec.push_back("13");

    svec.push_back("5");

    svec.push_back("/");

    svec.push_back("+");



    int ret = sln->evalRPN(svec);



    cout << ret << endl;



    return 0;

}