C++11 lambda 递归

#include <iostream>
#include <functional>

int main()
{
    auto f = [](int x) 
    {
        std::function<int(int)> s = [&](int n) 
        {
            return n == 1 ? 1 : n + s(n - 1);
        };
        return s(x);
    };
    std::cout << f(4) << std::endl;
}