#355 Leetcode Design Twitter Solution in C, C++, Java, JavaScript, Python, C# Leetcode

Algorithm

Problem Name: 355. Design Twitter

Problem Link: https://leetcode.com/problems/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.

Code Examples

#1 Code Example with C++ Programming

Code - C++ Programming


class Twitter {
private:
    vector<pair<int,int>>posts;
    unordered_map < int, unordered_map<int, int>>follows;
public:
    Twitter() {}
    
    void postTweet(int userId, int tweetId) {
        posts.push_back(make_pair(userId, tweetId));
    }
    
    vector<int> getNewsFeed(int userId) {
        vector<int>feed;
        int count = 0;
        for(int i = posts.size() - 1; i >= 0 && count  <  10; i--)
            if(posts[i].first == userId || follows[userId][posts[i].first])
                feed.push_back(posts[i].second), count++;
        return feed;
    }
    
    void follow(int followerId, int followeeId) {
        follows[followerId][followeeId] = 1;
    }
    
    void unfollow(int followerId, int followeeId) {
        follows[followerId][followeeId] = 0;
    }
};

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Input

–

cmd

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

Output

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cmd

[null, null, [5], null, null, [6, 5], null, [5]]

#2 Code Example with Java Programming

Code - Java Programming


import java.util.Map.Entry;


class Twitter {

  private final Map < Integer, Set userToTweet;
  private int timestamp;

  public Twitter() {
    this.userFollowing = new HashMap < >();
    this.timestamp = 0;
    this.userToTweet = new HashMap < >();
  }

  public void postTweet(int userId, int tweetId) {
    TweetNode tweetNode = new TweetNode(tweetId, this.timestamp++);
    if (this.userToTweet.containsKey(userId)) {
      tweetNode.next = this.userToTweet.get(userId);
    }
    this.userToTweet.put(userId, tweetNode);
  }

  public List < Integer> getNewsFeed(int userId) {
    PriorityQueue priorityQueue =
        new PriorityQueue<>((o1, o2) -> o2.timestamp - o1.timestamp);
    priorityQueue.addAll(
        this.userToTweet.entrySet().stream()
            .filter(
                entry ->
                    entry.getKey() == userId
                        || this.userFollowing
                            .getOrDefault(userId, new HashSet<>())
                            .contains(entry.getKey()))
            .map(Entry::getValue)
            .collect(Collectors.toList()));
    List < Integer> result = new ArrayList<>();
    while (!priorityQueue.isEmpty() && result.size() < 10) {
      TweetNode removed = priorityQueue.remove();
      result.add(removed.tweetId);
      if (removed.next != null) {
        priorityQueue.add(removed.next);
      }
    }
    return result;
  }

  public void follow(int followerId, int followeeId) {
    this.userFollowing.computeIfAbsent(followerId, k -> new HashSet < >()).add(followeeId);
  }

  public void unfollow(int followerId, int followeeId) {
    if (this.userFollowing.getOrDefault(followerId, new HashSet<>()).contains(followeeId)) {
      this.userFollowing.get(followerId).remove(followeeId);
    }
  }

  private static class TweetNode {
    int tweetId;
    int timestamp;
    TweetNode next;

    public TweetNode(int tweetId, int timestamp) {
      this.tweetId = tweetId;
      this.timestamp = timestamp;
    }
  }
}

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Input

–

cmd

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

Output

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cmd

[null, null, [5], null, null, [6, 5], null, [5]]

#3 Code Example with Javascript Programming

Code - Javascript Programming


const Twitter = function() {
  this.userTweets = new Map()
  this.userFollowing = new Map()
  this.lastIndex = 1
}

Twitter.prototype.postTweet = function(userId, tweetId) {
  let tweets = this.userTweets.get(userId)
  if (!tweets) {
    tweets = []
    this.userTweets.set(userId, tweets)
  }
  tweets.unshift({ id: tweetId, index: this.lastIndex })
  this.lastIndex = this.lastIndex + 1
}

Twitter.prototype.getNewsFeed = function(userId) {
  const followings = this.userFollowing.get(userId)
  let tweets = (this.userTweets.get(userId) || []).slice(0, 10)
  followings &&
    followings.forEach(uid => {
      if (uid === userId) return

      const userTweets = this.userTweets.get(uid)
      if (userTweets) {
        tweets = tweets.concat(userTweets)
      }
    })
  return tweets
    .sort((a, b) => b.index - a.index)
    .map(tweet => tweet.id)
    .slice(0, 10)
}

Twitter.prototype.follow = function(followerId, followeeId) {
  let followings = this.userFollowing.get(followerId)
  if (!followings) {
    followings = new Set()
    this.userFollowing.set(followerId, followings)
  }
  followings.add(followeeId)
}

Twitter.prototype.unfollow = function(followerId, followeeId) {
  const followings = this.userFollowing.get(followerId)
  followings && followings.delete(followeeId)
}

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Input

–

cmd

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

Output

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cmd

[null, null, [5], null, null, [6, 5], null, [5]]

#4 Code Example with Python Programming

Code - Python Programming


class Twitter:

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.tweets = collections.defaultdict(list)
        self.following = collections.defaultdict(set)
        self.order = 0
    def postTweet(self, userId, tweetId):
        """
        Compose a new tweet.
        :type userId: int
        :type tweetId: int
        :rtype: void
        """
        self.tweets[userId] += (self.order, tweetId), 
        self.order -= 1

    def getNewsFeed(self, userId):
        """
        Retrieve 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 herself. Tweets must be ordered from most recent to least recent.
        :type userId: int
        :rtype: List[int]
        """
        tw = sorted(tw for i in self.following[userId] | {userId} for tw in self.tweets[i])[:10]
        return [news for i, news in tw]
    

    def follow(self, followerId, followeeId):
        """
        Follower follows a followee. If the operation is invalid, it should be a no-op.
        :type followerId: int
        :type followeeId: int
        :rtype: void
        """
        self.following[followerId].add(followeeId)

    def unfollow(self, followerId, followeeId):
        """
        Follower unfollows a followee. If the operation is invalid, it should be a no-op.
        :type followerId: int
        :type followeeId: int
        :rtype: void
        """
        self.following[followerId].discard(followeeId)

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Input

–

cmd

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

Output

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cmd

[null, null, [5], null, null, [6, 5], null, [5]]

#5 Code Example with C# Programming

Code - C# Programming


using System.Collections.Generic;
using System.Linq;

namespace LeetCode
{
    public class _0355_DesignTwitter
    {
        private readonly Dictionary < int, User> users;
        private int timestamp = 0;

        /** Initialize your data structure here. */
        public _0355_DesignTwitter()
        {
            users = new Dictionary < int, User>();
        }

        /** Compose a new tweet. */
        public void PostTweet(int userId, int tweetId)
        {
            if (!users.ContainsKey(userId))
                users[userId] = new User(userId);
            users[userId].post(tweetId, timestamp++);
        }

        /** Retrieve 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 herself. Tweets must be ordered from most recent to least recent. */
        public IList < int> GetNewsFeed(int userId)
        {
            if (!users.ContainsKey(userId))
            {
                users[userId] = new User(userId);
                return new List<int>();
            }

            var followers = new HashSet < int>(users[userId].Followed);
            followers.Add(userId);
            var tweets = new SortedDictionary < Tweet, int>(Comparer.Create((a, b) => b.Time.CompareTo(a.Time)));
            foreach (var id in followers)
            {
                if (users[id].TweetHead != null)
                    tweets.Add(users[id].TweetHead, 1);
            }

            var count = 10;
            var results = new List < int>();
            while (tweets.Count > 0 && count > 0)
            {
                var t = tweets.Keys.First();
                results.Add(t.Id);
                tweets.Remove(t);

                if (t.Next != null)
                    tweets.Add(t.Next, 1);

                count--;
            }

            return results;
        }

        public void Follow(int followerId, int followeeId)
        {
            if (followerId == followeeId) return;

            if (!users.ContainsKey(followerId))
                users[followerId] = new User(followerId);
            if (!users.ContainsKey(followeeId))
                users[followeeId] = new User(followeeId);

            users[followerId].Follow(followeeId);
        }

        public void Unfollow(int followerId, int followeeId)
        {
            if (!users.ContainsKey(followerId))
                users[followerId] = new User(followerId);
            if (!users.ContainsKey(followeeId))
                users[followeeId] = new User(followeeId);

            users[followerId].Unfollow(followeeId);
        }

        public class Tweet
        {
            public Tweet(int id, int timestamp)
            {
                Id = id;
                Time = timestamp;
                Next = null;
            }

            public int Id { get; private set; }
            public int Time { get; private set; }
            public Tweet Next { get; set; }
        }

        public class User
        {
            public int Id { get; private set; }
            public HashSet < int> Followed { get; } = new HashSet<int>();
            public Tweet TweetHead { get; private set; }

            public User(int id)
            {
                Id = id;
            }

            public void Follow(int id)
            {
                Followed.Add(id);
            }

            public void Unfollow(int id)
            {
                Followed.Remove(id);
            }


            // everytime user post a new tweet, add it to the head of tweet list.
            public void post(int id, int timestamp)
            {
                Tweet newTweet = new Tweet(id, timestamp);
                newTweet.Next = TweetHead;
                TweetHead = newTweet;
            }
        }
    }

}

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Input

–

cmd

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

Output

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cmd

[null, null, [5], null, null, [6, 5], null, [5]]

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