https://leetcode.cn/problems/evaluate-reverse-polish-notation/description/

理解了逆波兰表达式的规则，按规则写就行，比较简单

int evalRPN(vector<string>& tokens) {
    stack<int> stk;

    for (string s : tokens) {
        if (s == "+") {
            int tmp = stk.top();
            stk.pop();
            stk.top() += tmp;
        } else if(s == "-"){
            int tmp = stk.top();
            stk.pop();
            stk.top() -= tmp;
        } else if(s == "*"){
            int tmp = stk.top();
            stk.pop();
            stk.top() *= tmp;
        } else if(s == "/"){
            int tmp = stk.top();
            stk.pop();
            stk.top() /= tmp;
        } else {
            stk.push(stoi(s));
        }
    }
    return stk.top();
}