150 Evaluate Reverse Polish Notation 逆波兰表达式求值

class Solution {
public:
    int evalRPN(vector<string> &tokens) {
        if(tokens.size() == 0)
            return 0;
        stack<int> stk;
        for(int i = 0; i < tokens.size(); ++i)
        {
            string s = tokens[i];
            if(s == "+" || s == "-" || s == "*" || s == "/")
            {
                if(stk.size() < 2)
                    return 0;
                int num2 = stk.top(); 
                stk.pop();
                int num1 = stk.top(); 
                stk.pop();
                int result = 0;
                 
                if(s == "+")
                    result = num1 + num2;
                else if(s == "-")
                    result = num1 - num2;
                else if(s == "*")
                    result = num1 * num2;
                else if(s == "/")
                    result = num1 / num2;
                stk.push(result);
            }
            else
            {
                stk.push(stoi(s));
            }  
        }
        return stk.top();
    }
};