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

        for(int i = 0; i < tokens.size(); i++){
            if(tokens[i] == "+" || tokens[i] == "-" || tokens[i] == "*" || tokens[i] == "/" ){
                int num1 = st.top();  st.pop();
                int num2 = st.top();  st.pop();
                if(tokens[i] == "+") st.push(num2 + num1);//注意：先弹出的是右操作数！！
                if(tokens[i] == "-") st.push(num2 - num1);
                if(tokens[i] == "*") st.push(num2 * num1);
                if(tokens[i] == "/") st.push(num2 / num1);
            }
            else{
                st.push(tokens[i]);
            }
        }
        return st.top();
    }
};