Evaluate Reverse Polish Notation

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        stack<int> nums;
        int num = tokens.size();
        int i=0;
        if(num == 1) return tonum(tokens[0]);
        while(i<num){
            while(i<num && !isop(tokens[i])){
                int tmp = tonum(tokens[i]);
                nums.push(tmp);
                i++;
            }
            int op2 = nums.top();
            nums.pop();
            int op1 = nums.top();
            nums.pop();
            if(tokens[i] == "+"){
                nums.push(op1+op2);
            }
            else if(tokens[i] == "*"){
                nums.push(op1 * op2);
            }
            else if(tokens[i] == "-"){
                nums.push(op1 - op2);
            }
            else if(tokens[i] == "/"){
                nums.push(op1 / op2);
            }
            i++;
        }
        return nums.top();
    }
    int tonum(string s){
        int num;
        stringstream ss(s);
        ss >> num;
        return num;
    }
    bool isop(string str){
        if(str.length() > 1) return false;
        if(str == "+") return true;
        if(str == "*") return true;
        if(str == "-") return true;
        if(str == "/") return true;
    }
};