Algorithm · Ebay · Easy
In a group chat, users can tag others by including mentions in message text. A mention begins with the '@' character and is immediately followed by one or more user IDs separated by commas, with no spaces anywhere inside the mention. For instance, "@id1" references a single user, while "@id1,id42,id999" references three users. Each user ID is made up of the prefix id and a positive integer from 1 to 999. A mention is only valid if it forms its own token: the '@' must appear…
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