10 Square (Block) Software Engineer (New Grad) Interview Questions (2026)
Square (now under Block) hires new-grad SWE through a recruiter screen, one OA, one technical phone screen, and a four-round virtual onsite. Coding rounds skew medium-hard with a focus on practical correctness. Block has multiple product brands (Square sellers, Cash App, TBD, Spiral) — the loop varies slightly by team.
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Loop overview
New-grad timeline reports show 5-7 weeks in 2026. Flow: recruiter → OA (HackerRank or CodeSignal, 60-90 min) → 45-min phone screen → four virtual onsite rounds. Onsite is typically two coding rounds, one design-leaning round, and one behavioral. Each Block brand has slight team-specific variations; Cash App tends to weight mobile experience, Square sellers weights payments knowledge.
Behavioral (3)
Why Square or Cash App? What interests you about payments or financial inclusion?
Frequently askedOutline
Block has a clear mission frame around financial empowerment, especially for the underbanked. Concrete reference: a Square seller you know, your own Cash App use, a specific product you find well-designed. Generic 'fintech is exciting' answers underperform.
Tell me about a time you had to design something for users very different from yourself.
Frequently askedOutline
STAR. Square's sellers and Cash App's users span demographics; engineers need real empathy. Pick a project where you considered user needs outside your default path. Show concrete decisions, not vague gestures.
Tell me about a time you fixed a hard bug in a system you did not originally write.
Frequently askedOutline
STAR. Show how you ramped on unfamiliar code: read it, ran it, instrumented it, asked questions. Block engineers inherit code constantly; they value engineers who can dive into unknown systems methodically.
Coding (LeetCode patterns) (4)
Given a list of strings, find the longest common prefix among them.
Frequently askedOutline
Vertical scan: for each character index, check if all strings have the same character at that index. Stop at first mismatch. O(n * k) where k is min string length. Alternative: divide-and-conquer or sort + compare first/last. Edge cases: empty list, single string, no common prefix.
Given two sorted arrays, find the median of the combined array in O(log(min(m,n))).
Occasionally askedOutline
Binary search on the smaller array. Partition both arrays so that left-halves combined equal right-halves combined in size, and max(leftA, leftB) <= min(rightA, rightB). The median sits at the partition boundary. Walk through carefully — this is one of the harder problems Square asks. Edge cases: arrays of very different sizes, even/odd combined length.
Implement a function that returns all valid IP addresses obtainable by inserting three dots into a digit string.
Occasionally askedOutline
Backtracking. Choose 1-3 digits for each segment, recurse on remainder, validate each segment (0-255, no leading zeros). When 4 segments placed and string consumed, emit. O(constant) — there are at most 81 partitions, each O(n) to validate. Edge cases: string too short or too long, all zeros.
Given a list of meetings as (start, end) pairs, return the minimum number of rooms required.
Frequently askedOutline
Sweep-line: separate starts and ends, sort both. Two pointers; for each event in time order, +1 room on start, -1 on end. Track max concurrent. O(n log n). Alternative: min-heap of end times — for each meeting, if heap-top end <= current start, reuse; else add a room.
Technical (3)
Implement a function that processes a refund. Given a transaction ID and an amount, validate and apply.
Frequently askedOutline
Steps: lookup transaction, validate (refund amount <= remaining refundable, transaction not voided, customer not banned), write refund record idempotently, debit merchant balance, credit customer. Discuss idempotency key, transactionality, error handling. Walk through edge cases: refund larger than original, refund after partial refunds, duplicate refund requests.
Design a peer-to-peer payment system (like Cash App). Walk through the components.
Frequently askedOutline
Components: user accounts, ledger, transfer service, notification service, fraud check, dispute handling. Discuss flow: user A initiates → fraud check → ledger debits A and credits B atomically → notify both. Discuss reversal, dispute, and abuse vectors. New-grad-friendly version — show structured thinking, not senior-level depth.
Explain how a card payment is processed from swipe to authorization.
Occasionally askedOutline
Card reader captures card data → encrypted to acquirer (Square in this case) → acquirer forwards to card network (Visa/MC) → network routes to issuing bank → issuer authorizes or declines → response flows back. Discuss settlement (later batch), interchange fees, and chargeback flow. Block engineers love this question — show conceptual depth on the payment ecosystem.
Square (Block) interview tips
- Block has multiple brands. Square (seller-side payments), Cash App (consumer payments), TBD/Spiral (decentralized). Ask the recruiter which team you are interviewing for — prep varies.
- Payments-system intuition helps. Be able to talk about authorization, settlement, refunds, chargebacks, and interchange at a conceptual level.
- Mission fit is real. Block's framing of financial empowerment matters; have a thoughtful, specific take.
- Idempotency, reconciliation, and double-spending prevention come up directly. Practice designing money-handling APIs.
- Block uses Kotlin heavily across mobile and backend, plus Java, Go, and Ruby on Rails (legacy and current). The interview accepts any language.
Frequently asked questions
How long is Square's SWE new-grad interview process in 2026?
Most reports show 5-7 weeks from OA to offer. Block's brand-specific recruiting can add timeline variance — confirm with your recruiter.
Does Square ask system design for new-grad SWE?
A lightweight design round is common, often payments-themed (P2P transfer, refund flow, fraud detection). New-grads show structured thinking, not senior-level designs.
Should I prep differently for Cash App vs Square sellers vs TBD?
The coding bar is the same across Block brands. Behavioral and design rounds tend to lean toward each brand's product domain — Cash App weights consumer payments, Square sellers weights merchant payments, TBD weights decentralized identity/payments.
What languages does Square use?
Kotlin and Java dominate mobile and backend. Ruby on Rails for parts of legacy and some current services. Go and Python for newer services. The interview accepts any language.
Is Square remote-friendly for new-grad SWE?
Block has been remote-first since 2020. Most roles are remote-eligible; some roles cluster in San Francisco, Atlanta, or New York. Confirm with your recruiter.
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