Graphsmedium45 minBackend Engineer

Idempotency Dependency DAG Recovery

Backend Engineer signal: topological sort + cycle detection in a payment idempotency context. This is a ProdMatch-owned backend engineer drill, framed as a April 2026 PhonePe Payments Core simulation, not a copied platform question.

Company context

PhonePe · Payments Core

Freshness

April 2026

Product surface

payment idempotency

ProdMatch interview simulation based on product-team patterns; not a claim of a real company question.

Question

You own payment idempotency for a Backend Engineer loop. Given requests and directed dependencies between them, return a valid execution order. If a cycle exists, return an empty list and identify that the rollout is blocked.

Input

n nodes labelled 0..n-1 and dependency pairs [before, after].

Output

A valid order of all nodes, or [] when dependencies contain a cycle.

Constraints

1 <= n <= 200000
0 <= dependencies.length <= 400000
The graph may be disconnected.

Concepts

idempotency
queues
rate limiting
topological sort
cycle detection
dependency graph

requests: 4, deps: [[0,1],[0,2],[2,3]] -> [0,1,2,3] or [0,2,1,3]

Approach

Try framing your own approach first. The 30 seconds you think before peeking is where learning happens.

Clean Solution

Reveal the approach first.

How well did you understand?

Your rating tunes when this problem shows up again.

Common Mistakes

Mark visited when enqueueing, not after repeated dequeue.
Do not ignore disconnected components if the product surface can have them.

Next Similar Problems

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