public class FriendshipService {
private Map<Integer, Set<Integer>> followers;
private Map<Integer, Set<Integer>> followings;
public FriendshipService() {
followers = new HashMap<>();
followings = new HashMap<>();
}
/*
* @param user_id: An integer
* @return: all followers and sort by user_id
*/
public List<Integer> getFollowers(int user_id) {
if (followers.containsKey(user_id)) {
return new ArrayList<Integer>(followers.get(user_id));
}
return new ArrayList<Integer>();
}
/*
* @param user_id: An integer
* @return: all followings and sort by user_id
*/
public List<Integer> getFollowings(int user_id) {
if (followings.containsKey(user_id)) {
return new ArrayList<Integer>(followings.get(user_id));
}
return new ArrayList<Integer>();
}
/*
* @param from_user_id: An integer
* @param to_user_id: An integer
* @return: nothing
*/
public void follow(int to_user_id, int from_user_id) {
followings.putIfAbsent(from_user_id, new TreeSet<Integer>());
followings.get(from_user_id).add(to_user_id);
followers.putIfAbsent(to_user_id, new TreeSet<Integer>());
followers.get(to_user_id).add(from_user_id);
}
/*
* @param from_user_id: An integer
* @param to_user_id: An integer
* @return: nothing
*/
public void unfollow(int to_user_id, int from_user_id) {
if (followings.containsKey(from_user_id))
followings.get(from_user_id).remove(to_user_id);
if (followers.containsKey(to_user_id))
followers.get(to_user_id).remove(from_user_id);
}
}
Notes
Notice here we used TreeSet to make sure the get methods return sorted and unique results.
