355. Design Twitter

Design a simplified version of Twitter where users can post tweets, follow/unfollow another user, and is able to see the 10 most recent tweets in the user's news feed.

Implement the Twitter class:

• Twitter() Initializes your twitter object.
• void postTweet(int userId, int tweetId) Composes a new tweet with ID tweetId by the user userId. Each call to this function will be made with a unique tweetId.
• List<Integer> getNewsFeed(int userId) Retrieves the 10 most recent tweet IDs in the user's news feed. Each item in the news feed must be posted by users who the user followed or by the user themself. Tweets must be ordered from most recent to least recent.
• void follow(int followerId, int followeeId) The user with ID followerId started following the user with ID followeeId.
• void unfollow(int followerId, int followeeId) The user with ID followerId started unfollowing the user with ID followeeId.

 

Example 1:

Input
["Twitter", "postTweet", "getNewsFeed", "follow", "postTweet", "getNewsFeed", "unfollow", "getNewsFeed"]
[[], [1, 5], [1], [1, 2], [2, 6], [1], [1, 2], [1]]
Output
[null, null, [5], null, null, [6, 5], null, [5]]

Explanation
Twitter twitter = new Twitter();
twitter.postTweet(1, 5); // User 1 posts a new tweet (id = 5).
twitter.getNewsFeed(1);  // User 1's news feed should return a list with 1 tweet id -> [5]. return [5]
twitter.follow(1, 2);    // User 1 follows user 2.
twitter.postTweet(2, 6); // User 2 posts a new tweet (id = 6).
twitter.getNewsFeed(1);  // User 1's news feed should return a list with 2 tweet ids -> [6, 5]. Tweet id 6 should precede tweet id 5 because it is posted after tweet id 5.
twitter.unfollow(1, 2);  // User 1 unfollows user 2.
twitter.getNewsFeed(1);  // User 1's news feed should return a list with 1 tweet id -> [5], since user 1 is no longer following user 2.

 

Constraints:

• 1 <= userId, followerId, followeeId <= 500
• 0 <= tweetId <= 104
• All the tweets have unique IDs.
• At most 3 * 104 calls will be made to postTweet, getNewsFeed, follow, and unfollow.

---

#https://leetcode.com/problems/design-twitter/

'''
1. 아이디어 :
    1)  follow는 해시맵 안에 해시셋, tweet은 deque(양방향 큐)를 만든다.
    postTweet : 제일 최신부터 저장하기 위해 deque의 왼쪽에 추가한다.
    getNewsFeed : deque를 앞에서부터 탐색하면서, userId가 follow한 사람이거나 자기 자신이면 ans에 추가한다.
    follow : HashMap[userId]에 followeeId를 추가한다.
    unfollow : HashMap[userId]에 followeeId를 제거한다. (try로 없을 시에 예외처리)
2. 시간복잡도 :
    1)  follow는 해시맵 안에 해시셋, tweet은 deque(양방향 큐)를 만든다.
    postTweet : O(1)
        deque 추가
    getNewsFeed : O(k)
        k는 트윗의 갯수. 최대 k번 탐색
    follow : O(1) * O(1)
        해시맵 검색 * 해시셋 추가
    unfollow : O(1) * O(1)
        해시맵 검색 * 해시셋 제거
3. 자료구조 :

'''
from collections import defaultdict
class Twitter:

    def __init__(self):
        self.follows = defaultdict(set)
        self.tweets = deque()

    def postTweet(self, userId: int, tweetId: int) -> None:
        self.tweets.appendleft([userId,tweetId])

    def getNewsFeed(self, userId: int) -> List[int]:
        count=0
        ans=[]
        while len(ans)<10 and count<len(self.tweets):
            if self.tweets[count][0] in self.follows[userId] or self.tweets[count][0] == userId:
                ans.append(self.tweets[count][1])
            count+=1
        return ans

    def follow(self, followerId: int, followeeId: int) -> None:
        self.follows[followerId].add(followeeId)

    def unfollow(self, followerId: int, followeeId: int) -> None:
        try:
            self.follows[followerId].remove(followeeId)
        except:
            pass