Python solution
IntuitionThe goal of this code is to track the number of mentions each user receives in an online-offline messaging system. Each user can be either online or of
# Intuition
The goal of this code is to track the number of mentions each user receives in an online-offline messaging system. Each user can be either online or offline, and users can send messages mentioning specific users, all users `ALL`, or only online users `HERE`. Mentions are only counted if the user being mentioned is online. Users go back online after being offline for 60 seconds.

# Approach
The approach involves initializing dictionaries to track user status and mentions, sorting events by timestamp and type, and then processing each event to update statuses and mentions. Users are initially marked as online with zero mentions. Events are sorted to prioritize `OFFLINE` events before `MESSAGE` events. As events are processed, offline users are checked and reactivated if they've been offline for more than 60 seconds. For `OFFLINE` events, the specified users are marked offline. For `MESSAGE` events, user mentions are updated based on whether they are mentioned individually, whether the message addresses `ALL` users, or only those `HERE` online. Finally, the mentions are collected into a list and returned. This ensures accurate tracking of user mentions based on their online status and event type.

# Complexity
- Time complexity:
  - 𝑂(𝑛log𝑛) due to sorting, where 𝑛 is the number of events.

- Space complexity:
  - 𝑂(𝑛) because of the storage required for the events.

# Code
```python3 []
class Solution:
    def countMentions(self, numberOfUsers: int, events: List[List[str]]) -> List[int]:
        # Need to keep track of offline users/online users
        # Need to keep track of timeStamp and status
        users_vs_status = {}
        mentions = {}

        events = sorted(events, key=lambda event: (int(event[1]), 0 if event[0] == "OFFLINE" else 1))
        for i in range(numberOfUsers):
            users_vs_status[i] = (1, 0) #0: offline, 1: online, intially all are online
            mentions[i] = 0

        for event in events:
            type = event[0]
            timestamp = int(event[1])
            mentions_string = event[2]

            ## update offline users that has surpassed 60 seconds
            for id in users_vs_status:
                status = users_vs_status[id][0]
                lasttime = users_vs_status[id][1]
                if(status == -1 and timestamp - lasttime >= 60):
                    status = 1
                    users_vs_status[id] = (1, timestamp)
                    
            if (type == "OFFLINE"):
                ids = [int(i) for i in event[2].split()]
                for id in ids:
                    status = users_vs_status.get(id)[0]
                    lasttime = users_vs_status.get(id)[1]
                    users_vs_status[id] = (-1, timestamp)

            elif(type == "MESSAGE" and mentions_string not in ["ALL", "HERE"]):
                ids = [int(i[2:]) for i in event[2].split()] # id1 id2 -> 1,2
                for id in ids:
                    status = users_vs_status.get(id)[0]
                    lasttime = users_vs_status.get(id)[1]
                    if(status==-1):
                        users_vs_status[id] = (status, lasttime)
                    else:
                        users_vs_status[id] = (status, timestamp)
                    
                    mentions[id] = mentions.get(id) + 1

            elif (type == "MESSAGE" and mentions_string ==  "ALL"):
                #loop over all keys , make them online, increase mentions
                for id in users_vs_status:
                    status = users_vs_status.get(id)[0]
                    lasttime = users_vs_status.get(id)[1]
                    if(status==-1):
                        users_vs_status[id] = (status, lasttime)
                    else:
                        users_vs_status[id] = (status, timestamp)
                    mentions[id] = mentions.get(id) + 1
    
            elif (type == "MESSAGE" and mentions_string ==  "HERE"):
                for id in users_vs_status:
                    status = users_vs_status.get(id)[0]
                    lasttime = users_vs_status.get(id)[1]
                    if (status == 1):
                        users_vs_status[id] = (status, timestamp)
                        mentions[id] = mentions.get(id) + 1
            
        return list(mentions.values())
```
['Python3']