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

class Solution {
public:
    int evalRPN(vector<string>& tokens) {
        stack<int> st;

        for(int i = 0; i != tokens.size(); i++){
            if( isNum(tokens[i]))
                st.push(stoi(tokens[i]));
            else if(tokens[i] == "+"){
                int num1 = st.top(); st.pop();
                int num2 = st.top(); st.pop();
                st.push(num1 + num2);
            }else if(tokens[i] == "-"){
                int num1 = st.top(); st.pop();
                int num2 = st.top(); st.pop();
                st.push(num2 - num1);
            }else if(tokens[i] == "*"){
                int num1 = st.top(); st.pop();
                int num2 = st.top(); st.pop();
                st.push(num1*num2);
            }else if(tokens[i] == "/"){
                int num1 = st.top(); st.pop();
                int num2 = st.top(); st.pop();
                st.push(num2/num1);
            }else{
                std::cout<<"Invalid token"<<std::endl;
            }
        }

        return st.top();
    }
private:
    bool isNum(string &s){
        int i;

        for(i = 0; i != s.size() && isspace(s[i]); i++)
            ;

        if(s[i] == '+' || s[i] == '-')
            i++;
        return isdigit(s[i]);
    }
};