PPractick

The problem

Build a tiny version of Twitter. Users can postTweet(userId, tweetId), follow(followerId, followeeId) and unfollow(...), and getNewsFeed(userId) returns the 10 most recent tweet IDs posted by the user or anyone they follow, newest first. Each user's own tweets are stored in time order, so a news feed is a k-way merge of already-sorted lists (the user plus their followees). A max-heap keyed by timestamp pulls the 10 newest without touching older tweets.

Example: postTweet(1, 5); getNewsFeed(1) → [5]; follow(1, 2); postTweet(2, 6); getNewsFeed(1) → [6, 5]; unfollow(1, 2); getNewsFeed(1) → [5].

Function signature

class Twitter {
public:
    void postTweet(int userId, int tweetId);
    vector<int> getNewsFeed(int userId);          // up to 10 tweet ids, newest first
    void follow(int followerId, int followeeId);
    void unfollow(int followerId, int followeeId);
};

Test cases

Operations	Output	Explanation
post(1,5) feed(1) follow(1,2) post(2,6) feed(1) unfollow(1,2) feed(1)	[5] [6, 5] [5]	LeetCode's example
user 1 posts tweets 1 to 11, then feed(1)	[11, 10, 9, 8, 7, 6, 5, 4, 3, 2]	Only the 10 newest
follow(1,1) post(1,7) feed(1)	[7]	Following yourself must not duplicate
post(2,3) follow(1,2) post(1,4) post(2,8) feed(1)	[8, 4, 3]	Interleaved by time
feed(9) for a user with nothing	[]

(Confirm: "recent" means posting order across all users; unfollowing someone you don't follow is a no-op; users always see their own tweets.)

How interviewers phrase it

• "Design a simplified version of Twitter where users can post tweets, follow or unfollow another user, and see the 10 most recent tweets in their news feed" (LeetCode 355).

Clarifying questions to ask

1. How many tweets in the feed? (10.)
2. Should users see their own tweets? (Yes.)
3. Which operations happen most often: reads or writes? (That decides between merging on read, "pull", and pushing to followers' feeds on write, "fan-out".)

Follow-ups to expect

1. "How do you order tweets globally?"
2. "How is the feed built?"
3. "Why not merge all tweets and sort?"
4. "What about celebrities with millions of followers?"
5. "What data structures per user?"

Related questions

• LRU Cache Medium
• Task Scheduler Medium
• Hand of Straights Medium
• Top K Frequent Elements Medium
• Program for Least Recently Used (LRU) Page Replacement Algorithm Medium
• LFU Cache Hard

All 474 dsa interview questions

DSA mock interviews · System design interview questions · Free mock interview practice · Browse interviewers · Sign in