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Count Mentions Per User

Difficulty: Medium

Problem Description

You are given an integer numberOfUsers representing the total number of users and an array events of size n x 3. Each events[i] can be either a message event indicating user mentions or an offline event indicating a user has gone offline. The task is to return an array mentions where mentions[i] represents the number of mentions the user with id i has across all MESSAGE events.

Key Insights

  • Users can be mentioned in messages even if they are offline.
  • There are three types of mentions in messages: specific user IDs, "ALL" for all users, and "HERE" for online users only.
  • Users go offline for 60 time units, after which they are automatically back online.
  • The processing of offline events occurs before any message events at the same timestamp.

Space and Time Complexity

Time Complexity: O(n * m) where n is the number of events and m is the average number of mentions in a message event.
Space Complexity: O(u) where u is the number of users, to maintain the mentions count and online status.

Solution

To solve the problem, we will use:

  • An array to keep track of mentions for each user.
  • A boolean array to track the online/offline status of each user.
  • We will iterate through the events, processing offline events first to update user statuses before handling message events.
  • For message events, we will parse the mention string and update the mentions count based on the parsed user IDs, "ALL", or "HERE".

Code Solutions

def countMentions(numberOfUsers, events):
    mentions = [0] * numberOfUsers
    online_status = [True] * numberOfUsers
    offline_until = [0] * numberOfUsers

    for event in events:
        event_type, timestamp, detail = event

        if event_type == "OFFLINE":
            user_id = int(detail[2:])
            online_status[user_id] = False
            offline_until[user_id] = int(timestamp) + 60

        elif event_type == "MESSAGE":
            # Process offline events before message
            for user_id in range(numberOfUsers):
                if not online_status[user_id] and int(timestamp) >= offline_until[user_id]:
                    online_status[user_id] = True
            
            # Count mentions
            if detail == "ALL":
                for i in range(numberOfUsers):
                    mentions[i] += 1
            elif detail == "HERE":
                for i in range(numberOfUsers):
                    if online_status[i]:
                        mentions[i] += 1
            else:  # specific user mentions
                mentioned_users = detail.split()
                for user in mentioned_users:
                    user_id = int(user[2:])
                    mentions[user_id] += 1

    return mentions
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