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

int evalRPN(vector<string> &tokens) {
    int sum = 0;
    stack<int>st;
    int res = 0;
    for(int i=0; i<tokens.size(); i++){
        if(tokens[i] != "+" && tokens[i] != "-"
           && tokens[i] != "*" && tokens[i] != "/"){
            st.push(atoi(tokens[i].c_str()));    
        }
        else{
            int b = st.top();st.pop();
            int a = st.top();st.pop();
            
            int c = calResult(a,b,tokens[i]);
            st.push(c);
        }
    }
    return st.top();
}
private:
int calResult(int a, int b, string& symbol){
    if(symbol == "+") return a+b;
    else if(symbol == "-") return a-b;
    else if(symbol == "*") return a*b;
    else if(symbol == "/") return a/b;
}