Greedymedium45 minBackend Engineer

Idempotency Greedy Load Rebalancer

Backend Engineer signal: greedy proof + sorting in a payment idempotency context. This is a ProdMatch-owned backend engineer drill, framed as a March 2026 Cars24 Payments Core simulation, not a copied platform question.

Company context

Cars24 · Payments Core

Freshness

March 2026

Product surface

payment idempotency

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

Question

Rebalance requests across shards for payment idempotency. Each move has a cost equal to distance between shards. Compute minimum cost to make shard loads differ by at most one.

Input

Array of shard loads.

Output

Minimum movement cost.

Constraints

1 <= shards <= 200000
Total load <= 1e12

Concepts

idempotency
queues
rate limiting
greedy proof
sorting
load balancing

loads [0,3,0] -> move one unit left and one right, cost=2

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?

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Common Mistakes

A greedy idea without a proof may fail hidden cases.
Do not mutate sorted keys in a way that breaks ordering.

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