1. What is the output of the following C++ program?

#include <algorithm>
#include <iostream>
#include <list>

struct P
{
    bool operator()(const int &n) const
    {
        return n % 3 == 0;
    }
};

int main()
{
    std::list<int> l({ 5, 2, 6, 1, 13, 9, 19 });
    std::cout << l.size();
    std::remove_if(l.begin(), l.end(), P());
    std::cout << l.size() << std::endl;

    return 0;
}