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Reddit Coding Interview Questions (2026): 10 Prompts + How to Answer

Reddit coding / dsa practice is one slice of a 50-prompt set. Restate the input, name a brute-force baseline, then improve it. Say time and space before you claim you are done. Say who the moderator is in your design. A feed that ignores them will get a follow-up.

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All Behavioral Coding / DSA System design Role-specific Culture & values

Coding / DSA

Commonly reported

Reddit coding theme to practice: given a stream of vote events, return the most frequent few without sorting the whole history. State time and space, then name one edge case.

How to approach. Repeat the input and output in your own words, then give the naive cost. Only then reach for a better structure for vote events.

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Coding / DSA

Commonly reported

Practice prompt in a Reddit coding round: detect whether a sequence of comment tree ids repeats in a cycle, and describe the smallest window that proves it. Explain the invariant before you code.

How to approach. Write the invariant on the side before the loop. Reddit rounds go better when the interviewer can see what stays true.

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Coding / DSA

Commonly reported

Commonly discussed Reddit data-structure theme: merge overlapping ranges drawn from subreddit rule versions and report the covered span. Talk through a correct pass, then a tighter one.

How to approach. Dry-run three cases: empty, one element, and the boundary you are afraid of. Talk while you do it.

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Coding / DSA

Commonly reported

Representative Reddit graph prompt: find a path across mod action logs when some links are closed. Say which search you would use and why.

How to approach. Name the search or traversal and the reason. If you switch approaches, say what the first one failed to handle.

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Coding / DSA

Commonly reported

Reddit coding practice: design an iterator over feed ranking features that stays correct while new items arrive. Cover an empty start and a burst in the middle.

How to approach. If new data arrives while you iterate, say whether you snapshot or follow updates. Pick one and stick to it.

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Coding / DSA

Commonly reported

Practice this Reddit style prompt: keep a running median over the latest window of award grants. Explain the two structures you would hold and their costs.

How to approach. For a running aggregate, say the window, the update cost, and what you evict. A diagram of two structures is enough.

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Coding / DSA

Commonly reported

Coding theme associated with Reddit interviews: group report queue items that share a signature. Describe the hash you would explain out loud and what collision means here.

How to approach. Explain the key you hash and what a collision means for this data. Do not jump to library names.

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Coding / DSA

Commonly reported

Reddit scheduling-style prompt: assign ad placement requests to a fixed pool of workers so the finish time is as small as you can justify. Start with a greedy idea and say where it fails.

How to approach. Start greedy, then show one input where greedy is wrong if it is wrong. That correction is the answer.

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Coding / DSA

Commonly reported

Representative Reddit search prompt: locate an id inside a rotated, mostly sorted list of user karma snapshots. Narrate the boundary checks before you write the loop.

How to approach. Narrate the low and high bounds before the first iteration. Off-by-one is the usual miss on rotated data.

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Coding / DSA

Commonly reported

Reddit coding theme: admit a burst of crosspost links only while each key stays under a fixed per-minute budget. Define the key, the window, and how you would test a boundary.

How to approach. Define the identity you rate-limit, the clock, and the response when the budget is spent. Test the exact boundary.

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