import java.util.*;

public class SparseGraph {
    private List<Set<Integer>> neighbours;

    public SparseGraph(int n) {
        neighbours = new ArrayList<>();
        for (int i = 0; i < n; i++)
            neighbours.add(new HashSet<>());
    }

    public boolean hasEdge(int p, int q) {
        return neighbours.get(p).contains(q);
    }

    public Set<Integer> edgesFrom(int p) {
        return neighbours.get(p);
    }

    public void addEdge(int p, int q) {
        neighbours.get(p).add(q);
        neighbours.get(q).add(p);
    }
}