std::string& trim(std::string &);

int main()
{
    std::string s = " Hello World!! ";
    std::cout << s << " size:" << s.size() << std::endl;
    std::cout << trim(s) << " size:" << trim(s).size() << std::endl;

    return 0;
}

std::string& trim(std::string &s)
{
    if (s.empty())
    {
        return s;
    }

    s.erase(0,s.find_first_not_of(" "));
    s.erase(s.find_last_not_of(" ") + 1);
    return s;
}