#include <stdexcept>
#include <iostream>
#include <list>

class TrainComposition
{
private:
	std::list<int> trains;
public:

	void attachWagonFromLeft(int wagonId)
	{
		trains.push_front(wagonId);
	}
	
	void attachWagonFromRight(int wagonId)
	{
		trains.push_back(wagonId);
	}

	int detachWagonFromLeft()
	{
		auto temp = trains.front();
		trains.pop_front();
		return temp;
	}

	int detachWagonFromRight()
	{
		auto temp = trains.back();
		trains.pop_back();
		return temp;
	}
};

#ifndef RunTests
int main()
{
	TrainComposition tree;
	tree.attachWagonFromLeft(7);
	tree.attachWagonFromLeft(13);
	std::cout << tree.detachWagonFromRight() << "\n"; // 7 
	std::cout << tree.detachWagonFromLeft(); // 13
	return EXIT_SUCCESS;
}
#endif