Pin Keyword Alert

Pinterest wants to send an alert when a newly created pin matches a user's keyword interests.

Given:

• pins: a list of pins, each with id, description, and username
• users: a list of usernames who should receive alerts
• keywords: a list of keywords to match (case-insensitive substring match against the description)

Return a list of pin IDs (in input order) that should trigger an alert. A pin triggers an alert when:

1. Its username is in the users list, AND
2. Its description contains any of the keywords as a whole-word substring, ignoring punctuation.

Whole-word definition: the keyword must appear bordered by non-letter characters (or string boundaries). The match is case-insensitive.

Examples:

pin description: "I love candy and cake" keywords=["candy"] -> match pin description: "I have a candle" keywords=["can"] -> NO match (substring, not a whole word) pin description: "Eat candy! Now" keywords=["candy"] -> match (punctuation is a non-letter boundary)

Follow-up Questions:

• What if there are millions of pins streaming in per minute and tens of thousands of keywords?
• How would you support stemming (so "running" matches "run")?