class DER {
public:
	DER(int max);
	virtual	~DER();

	void join(int e1, int e2);
	int equiv(int e1, int e2);

private:
	int root(int e);

	const int N;
	int *r;		//inverse tree
	int *s; 	//size of tree
	//Representation invariant:
	// I1: (forall i: : "r[i] is the root of i")
	// I2: (forall i: : r[i]=i => "s[i] is the size of the tree with i as the root")
};