TL;DR — Reddit Comment System

Design Reddit's comment system — a deeply nested, threaded discussion platform where comments can have unlimited levels of replies, each independently voted on and ranked. A popular AMA (Ask Me Anything) post might accumulate 50,000 comments in a tree structure 20 levels deep, and the system must render the most relevant comments first while allowing users to expand collapsed threads on demand. Key features: Post comments as replies to a post or to other comments (unlimited depth). Upvote and downvote individual comments.