std::accumulate(std::begin(x), std::end(x), string(), [](string &ss, string &s){return ss.empty() ? s : ss + " " + s;})
- ss是初始值 string(),相當於上一個值
- s爲當前遍歷的元素
std::accumulate(std::begin(x), std::end(x), string(), [](string &ss, string &s){return ss.empty() ? s : ss + " " + s;})
https://stackoverflow.com/questions/419163/what-does-if-name-main-do