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;}