Support follow & unfollow, getFollowers, getFollowings.
Note: the results of getfollow() are sorted.
Example
follow(1, 3)
getFollowers(1) // return [3]
getFollowings(3) // return [1]
follow(2, 3)
getFollowings(3) // return [1,2]
unfollow(1, 3)
getFollowings(3) // return [2]思路：TreeSet是已经排好序的set，直接用treeset去收集就可以了；
public class FriendshipService {
private HashMap<Integer, Set<Integer>> followers, followings;
public FriendshipService() {
this.followers = new HashMap<Integer, Set<Integer>>();
this.followings = new HashMap<Integer, Set<Integer>>();
}
/*
* @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>();
}
return new ArrayList<Integer>(followers.get(user_id));
}
/*
* @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>();
}
return new ArrayList<Integer>(followings.get(user_id));
}
/*
* @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) {
if(!followers.containsKey(to_user_id)){
followers.put(to_user_id, new TreeSet<Integer>());
}
followers.get(to_user_id).add(from_user_id);
if(!followings.containsKey(from_user_id)){
followings.put(from_user_id, new TreeSet<Integer>());
}
followings.get(from_user_id).add(to_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(followers.containsKey(to_user_id)){
followers.get(to_user_id).remove(from_user_id);
}
if(followings.containsKey(from_user_id)){
followings.get(from_user_id).remove(to_user_id);
}
}
}