A Software Engineer at Opto Investments serves as a critical architect of the firm's mission to modernize private market access. You will contribute to a sophisticated platform that bridges the gap between complex financial products and the investors seeking to access them. Your work directly impacts how financial data is processed, visualized, and delivered, requiring a balance of high-level systems thinking and precise, performant code.
This role is inherently cross-functional, placing you at the intersection of product strategy and engineering execution. You will face challenges involving data-intensive workflows, real-time API integrations, and the development of user-facing interfaces that demand both reliability and elegance. Because Opto Investments operates in a highly regulated and high-stakes environment, the software you build must be robust, scalable, and secure.
You will be expected to demonstrate not just coding proficiency, but a genuine interest in the financial domain and the specific product challenges Opto Investments is solving.
Initial Screening
reportedBefore anything technical happens, someone has to decide which rung of the ladder your loop is calibrated to, and that decision sets the bar for every round after it. It comes from how you describe scope, not from your title, because titles do not convert cleanly between companies. The weak version of the answer is team size and years. The strong version names the largest change you shipped where nobody reviewed the design, what would have broken if you had been wrong, and what you were paged for. Get the level said out loud on this call, because the range and the loop both follow from it.
What to demonstrate
- Whether the scope in your own account maps onto a level the team actually has an opening at, so a mismatch ends the process cheaply rather than after four interviewers have spent a day
- Whether your title needs re-mapping: the same word describes very different amounts of independent decision-making at a twenty-person company and a ten-thousand-person one
- Whether your compensation expectation can be filled at that level in the structure the role pays in, which is why the number gets asked for before any engineer is scheduled
How to prepare
- Write down two changes from the last two years: the largest one you designed with nobody reviewing the design, and the largest one where someone more senior did. Lead with the first when scope comes up, and be ready to say which parts of the second were yours
- Ask which level the loop is calibrated to and what changes at the level above it, then plan your weeks from that answer rather than from the posting
- Settle a total-compensation range beforehand with the split named, base against bonus against equity and its vesting period, so a question about numbers gets a number instead of the word market
Hands-on Technical Evaluation
reportedMost of the time lost in this format is not lost to thinking. It goes to a standard-library call you half-remember, an off-by-one in a loop bound, and a debugging loop that mutates code at random until something passes. When output is wrong, stop re-reading the whole function: take the smallest input that reproduces it and walk the state through by hand, printing intermediates if the environment allows. Guessing at a fix without a failing case you understand is how a five-minute bug becomes twenty, and the clock does not pause while you do it.
What to demonstrate
- Whether you reach the right structure without a detour, and can write it from memory rather than only recall that one exists
- Whether overflow is considered where the language has fixed-width integers, since a signed 32-bit value stops at 2,147,483,647 and then wraps in Java, is undefined behaviour in C++, and does not arise in Python, whose integers grow instead
- Whether recursion depth is treated as a constraint on large inputs, given that CPython's default limit is 1000 frames and a deep recursion can exhaust the stack in any language where an iterative version would not
- Whether a failing case is isolated and explained before any edit is made to the code
How to prepare
- From an empty file and with no references open, implement the pieces you lean on most: a heap push and pop, an iterative DFS with an explicit stack, and a binary search whose midpoint is written lo + (hi - lo) / 2, which avoids the overflow that (lo + hi) / 2 can hit in a fixed-width integer type
- Time yourself on the ten library calls you look up most, such as sorting with a custom comparator, splitting and joining strings, and finding the next key at or above a value in an ordered map, until the lookup is gone
- Take a solution you know is broken and, before touching it, write one sentence naming the input, the expected value and the actual value. Repeat until you do it without deciding to.
Power Day Assessment
reportedWhat this round decides is narrow: whether you can produce code that runs and is correct on inputs nobody showed you. An elegant solution that does not compile scores below a plain one that does, so write a correct brute force first, say out loud that you know its cost, and improve it with the working version still on screen. What separates strong answers is who finds the broken case. Trace your own code against an empty input, a single element, and duplicate keys before you say you are finished, because being told is far more expensive than noticing.
What to demonstrate
- Whether degenerate inputs get checked without being asked for: an empty collection, one element, every element equal, and the extreme value the input type allows
- Whether the complexity you state matches the code you actually wrote, including a sort or a copy sitting inside a loop
- Whether the finished answer is verified against the worked examples before you call it done, rather than assumed correct because the code reads correctly
How to prepare
- Take five problems you have already solved and, without running anything, write down what each returns for empty input, a single element, and all-duplicates. Then run them and count how many you predicted wrong.
- Drill the brute force as its own skill: on ten problems, write only the obviously-correct slow version and time how long it takes to get it passing. If that is more than a few minutes, that is what to practise, not the optimal version.
- Add a fixed last step before you submit anything, reading only the loop bounds and the initial value of each accumulator, which is where most off-by-one errors live
PracHub editorial advice for the preparation topics above.
Treating money as a decimal with two places
ISO 4217 exponents are 0 for currencies such as JPY and KRW, 2 for most, and 3 for BHD, KWD, JOD, OMR and TND, so a hard-coded multiply-by-100 is off by a factor of 100 or 10 depending on the currency, in opposite directions. Floating point is worse: IEEE 754 binary64 cannot represent 0.1 exactly, so repeated accrual accumulates drift that appears as a handful of minor units in the daily reconciliation and then gets 'fixed' by widening the match tolerance, which is how a genuine break becomes invisible. The only forms that survive a reconciliation are integer minor units with the exponent carried alongside the currency code, or a fixed-scale decimal type with exactly one documented rounding point.
Calling a compensating action a rollback
A saga's compensation is a new, externally visible business event, not an undo. A refund after a capture leaves both movements on the customer's statement, may not return the scheme fee, and lands days later rather than immediately. Designing a multi-service flow as though the compensation restores the prior state produces flows that turn out to be unimplementable at the final step, when the thing that needs undoing has already left the building. The sequence has to be ordered so the irreversible step is last and the reversible ones precede it, with an explicit pending state shown to the customer while a compensation is in flight.
Tests that assert on the implementation rather than the behaviour
Assert on what a caller can observe, not on the number of internal calls or the shape of a private field. A test that breaks on every refactor but still passes when the answer is wrong costs more than it protects.
Naming no test cases at all
State what you would test before being asked: empty input, a single element, all elements equal, the maximum permitted size, and the input that exercises the branch you just wrote. It costs thirty seconds and is much of what separates someone who has shipped code from someone who has only solved puzzles.
Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.
How do you approach optimizing code for performance during a live-codi…
How do you approach optimizing code for performance during a live-coding session?
Approach
- Name the brute-force solution and its complexity before improving on it.
- State the target complexity and say which constraint rules the naive version out.
- Restate the input: its shape, its size, and what is guaranteed about it.
Follow-up
- How does this change if the input no longer fits in memory?
- Which test case would catch an off-by-one here?
How would you implement a function to convert Roman numerals into inte…
How would you implement a function to convert Roman numerals into integers?
Approach
- Walk one small example through your approach before writing the whole thing.
- Restate the input: its shape, its size, and what is guaranteed about it.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- Which test case would catch an off-by-one here?
Can you write a script to identify and remove duplicate rows from a CS…
Can you write a script to identify and remove duplicate rows from a CSV file while answering analytical questions about the data?
Approach
- Choose the data structure from the access pattern, not from familiarity.
- Name the brute-force solution and its complexity before improving on it.
- State the target complexity and say which constraint rules the naive version out.
Follow-up
- How does this change if the input no longer fits in memory?
- What is the worst case, and how likely is it on real data?
Can you design a solution that takes user input, triggers an API call,…
Can you design a solution that takes user input, triggers an API call, and dynamically renders the returned data?
Approach
- Walk one small example through your approach before writing the whole thing.
- Restate the input: its shape, its size, and what is guaranteed about it.
- Choose the data structure from the access pattern, not from familiarity.
Follow-up
- What is the worst case, and how likely is it on real data?
- Which test case would catch an off-by-one here?
Resolve party identity across merges without rewriting posted history
Duplicate parties get merged over time: a stream of up to 5 million (loser_party_id, winner_party_id, merged_at) events, deliverable in any order, sometimes repeating a pair already merged. Ten million account rows and hundreds of millions of ledger_entry rows already reference pre-merge party ids and must not be rewritten. Build the structure that answers 'what is the canonical party for this id?' in near-constant time, and explain how a report that groups by party stays correct without touching a single historical row.
Approach
- The merge events form an undirected graph over party ids and the question is connected components, so use disjoint-set union with union by size and path compression:
findandunionamortise to O(alpha(n)), and alpha(n) is at most 4 for any n that exists. Five million merges over ten million ids is two integer arrays and a few hundred milliseconds. - The domain twist: the representative cannot be whichever root union-by-size happens to pick, because the canonical party is a business decision recorded as
winner_party_id. Keep union by size for the tree shape and a separateroot -> canonical_party_idmap for the label. Conflating them makes the canonical id flip when an unrelated merge reshapes the tree, and every report built on it moves retroactively. - Never rewrite
ledger_entryoraccount. Resolve at read through the canonical map: rewriting posted rows destroys the audit trail, locks hundreds of millions of rows, and buys nothing the map does not already give you. - Materialise the map fully expanded as
party_id -> canonical_party_id, one row per member rather than per edge, so a report is a single join instead of a recursive traversal. Rebuild it from the event log, which keeps it a derived artefact that can be regenerated rather than a second source of truth that drifts. - Pin down determinism: a repeated merge is a no-op because both ids already share a root, but two events merging the same pair in opposite directions is a genuine conflict. Write the tiebreak down — lowest
merged_at, then lowest id — or two replays of the same log produce different labels for the same data. - Un-merges: disjoint-set union has no delete. If a merge can be reversed, keep the event log authoritative and rebuild the whole structure from the surviving events; at 5 million events that is seconds, and far cheaper than any incremental un-union scheme.
Worked solution 25 min
- Implement disjoint-set union with
parentandsizearrays plus aroot -> canonicalmap, keepingfinditerative with path halving so a ten-million-element chain cannot exhaust the stack. - Apply events sorted by
(merged_at, loser_party_id), setting the new root's canonical label to that event'swinner_party_id. - Fixture: A into B, C into D, B into D applied in that order, plus a repeat of A into B and a self-merge of B into B.
- Expand to the flat map and assert all of A, B, C and D resolve to the same canonical id.
- Shuffle the event order, re-run, and assert the flat map is identical — this is the test that catches an order-dependent canonical label.
Follow-up
- A merge is reversed after six months of postings. What exactly gets rebuilt, and what does the statement produced last month now say?
- Two processes apply merge events concurrently. What guarantees they converge on an identical canonical map?
- Resolving at read on the hot path versus materialising the map — where does that join actually live, and what does each cost?
Store multi-currency amounts and prove each transaction balances
ledger_entry holds entry_id, transaction_id, account_id, direction (debit or credit), amount_minor (bigint, CHECK > 0), currency char(3), source_type, source_id, business_date and posted_at. The service posts in JPY, USD and KWD. Specify the column types for money and the reference table that carries each currency's ISO 4217 minor-unit exponent, and say why that exponent must not be a constant in application code. Then write the query that returns every transaction_id whose entries fail to sum to zero within each currency, ordered by the largest absolute imbalance.
Approach
- State the representation: amount_minor is an integer count of minor units and the scale lives with the currency, not in the column. int4 tops out at 2,147,483,647 minor units, which is about 21.5 million units of a 2-exponent currency, so bigint, and the currency code travels on every row that carries an amount.
- Normalise the scale into currency(code char(3) primary key, exponent smallint, name) and join it only at display and parse boundaries. The exponent is 0 for JPY and KRW, 2 for USD and EUR, 3 for KWD, BHD, JOD, OMR and TND, so a hard-coded divide by 100 is wrong by 100x for JPY and by 10x for KWD, in opposite directions.
- Rule out binary floating point outright: IEEE 754 binary64 cannot represent 0.1, so repeated accrual drifts by a few minor units that later appear as reconciliation breaks. If a fixed-scale decimal is used instead, pin the scale per currency and name the single place rounding happens.
- Write the balance check as a grouped aggregate: sign the amount with a CASE on direction, GROUP BY transaction_id, currency, and HAVING the signed sum <> 0. The sign belongs in direction, and the CASE is the one place it becomes arithmetic.
- Then order in a second layer, because the ordering the question asks for cannot be written on the grouped query itself. PostgreSQL lets an output alias stand alone in ORDER BY but resolves anything inside an expression against the input columns, so ORDER BY abs(imbalance_minor) DESC raises SQLSTATE 42703, column "imbalance_minor" does not exist, while the bare ORDER BY imbalance_minor DESC is accepted and sorts signed rather than absolute. Wrap the grouped query and order by abs() outside it, or repeat the aggregate as ORDER BY abs(SUM(CASE ...)) DESC, which is legal because aggregates are allowed there.
- Group per (transaction_id, currency) rather than per transaction, because a cross-currency movement balances only within each currency, bridged by an explicit FX position account whose rate, source and timestamp are stored on the transaction.
- Note that no CHECK constraint can express this, since it spans rows: enforce it at write time with a DEFERRABLE INITIALLY DEFERRED constraint trigger that fires at commit, or by routing every posting through one function, and keep this query as the independent audit.
Follow-up
- A transaction has a USD leg and a JPY leg. Which accounts appear, and where does the rounding residual land?
- How would you enforce the zero-sum rule at write time without serialising all postings behind one lock?
- Someone proposes storing a display amount alongside amount_minor. What goes wrong at the first rate change or rounding rule change?
Add and backfill business_date on a live ledger table
ledger_entry holds 4 billion rows, is append-only, takes 10,000 inserts per second, and every reporting query currently derives the business date as posted_at::date. You must add business_date date NOT NULL, populated from the cutoff rule (17:00 in the account's own timezone), backfilled across all history, indexed, and cut over, with no write downtime and no long-held lock. Give the ordered migration steps with the lock each one takes, how you make the backfill restartable and throttled, and how you retire the old expression safely.
Approach
- Add the column nullable and with no default. ALTER TABLE ... ADD COLUMN takes ACCESS EXCLUSIVE but is a catalogue-only change held for microseconds. The hazard is the lock queue, not the statement: a blocked ALTER waits behind one long reader holding ACCESS SHARE, and every query arriving afterwards queues behind the ALTER's pending ACCESS EXCLUSIVE, so set lock_timeout to a couple of seconds and retry rather than letting a metadata change take the table down.
- Deploy the write path before the backfill, so new inserts populate business_date from the cutoff rule while reads stay on the old expression. The backfill then chases a closed set with a fixed upper bound instead of a moving target.
- Backfill in bounded batches keyed by entry_id range, on the order of 50,000 rows per statement, committing between batches and recording the high-water mark in its own table so a killed run resumes instead of restarting. WHERE business_date IS NULL makes each batch idempotent, and the pacing is set by replica lag and dead-tuple growth rather than CPU, since each UPDATE writes a new row version and the WAL volume is proportional to the rows touched.
- Install the constraint without a blocking scan: ALTER TABLE ... ADD CONSTRAINT ck_business_date CHECK (business_date IS NOT NULL) NOT VALID takes a brief ACCESS EXCLUSIVE and scans nothing, then VALIDATE CONSTRAINT takes SHARE UPDATE EXCLUSIVE and runs alongside reads and writes. On PostgreSQL 12 and later, SET NOT NULL can then use the validated CHECK and skip its own full scan; on 11 and earlier it always scans, so the CHECK is the migration on those versions.
- Build the index with CREATE INDEX CONCURRENTLY, which avoids ACCESS EXCLUSIVE at the cost of two table passes, cannot run inside a transaction block, and on failure leaves an INVALID index that must be dropped and rebuilt rather than reused.
- Cut over behind a flag: run the new and old expressions side by side for one reporting cycle and compare totals per day, since the cutoff rule will legitimately move entries near 17:00 across the boundary. Only once they reconcile do you retire the posted_at::date expression index, and you keep posted_at as the ordering key rather than repurposing it.
Worked solution 40 min
- Write the migration as numbered SQL statements, annotating each with its lock mode and expected duration, and set lock_timeout plus a retry around every ALTER.
- Rehearse on a 10-million-row copy under a concurrent insert load, measuring per-batch duration, WAL generated and replica lag.
- Kill the backfill at a random point, restart it, and confirm it resumes from the high-water mark.
- Add the CHECK as NOT VALID, VALIDATE it while inserts continue, then SET NOT NULL and build the index CONCURRENTLY.
- Run old and new date expressions side by side for one cycle and diff the daily totals before dropping the old expression index.
Follow-up
- Reporting now wants ledger_entry partitioned by business_date. Why can this not be another ALTER, and what is the migration instead?
- Nightly totals move for the days around the cutoff change. How do you tell a correct restatement from a backfill bug?
- The backfill is halfway done when a replica falls 20 minutes behind. What do you throttle, and what do you refuse to throttle?
Specify the idempotency key contract for payment creation
POST /payments takes an idempotency key header plus amount_minor, currency, merchant and instrument token. The caller is a merchant server that retries on timeout and may retry concurrently from two workers. Storage is idempotency_key (id, scope text, key text with UNIQUE (scope, key), request_fingerprint bytea, status enum in_progress/completed/failed, response_status smallint, response_body jsonb, locked_at, completed_at, expires_at). Specify the scope, what the fingerprint covers, the response when the same key arrives with a different body, the response while the first request is still in flight, and what a replay after expires_at does.
Approach
- Scope the key to caller plus operation, for example 'merchant:1234:create_payment'. A global key space lets two merchants collide, and a key scoped only to the merchant lets a key used on one endpoint replay that endpoint's stored response for a different operation.
- Fingerprint the canonicalised body: sorted keys, no insignificant whitespace, amounts as integer minor units. Cover the fields that determine the effect and deliberately exclude client metadata such as a trace id, so a retry that adds one is still recognised as a retry. Same key with a different fingerprint is 409 with its own code, never the cached response and never the new effect.
- Win the concurrent race in the database: INSERT ... ON CONFLICT DO NOTHING RETURNING id. An empty return means this request lost, so it reads the winner's row. A SELECT-then-INSERT is not a fix; under READ COMMITTED both workers take fresh snapshots and both pass the check.
- Define in_progress explicitly. The loser reads status in_progress and receives 409 with a retry hint rather than a blocking wait or a second attempt. Note that Retry-After is defined for 429, 503 and 3xx, so on a 409 it is an extension you document rather than something the caller's HTTP stack already honours. A locked_at older than the lease is reclaimable only through a conditional UPDATE that matches the old locked_at, otherwise two reclaimers both proceed.
- Commit the key row before calling the processor, so a crash leaves evidence of an attempt that can be queried and converged. On completion store response_status and response_body and replay them verbatim, including the original status code.
- State the retention window (24 hours is a common choice) and its consequence: after expires_at the same key is a new request, so the caller's total retry deadline must sit inside the window or the mechanism silently stops protecting anything. The header name comes from an IETF draft rather than a ratified RFC, so document it as your convention.
Worked solution 30 min
- Implement the endpoint over PostgreSQL with UNIQUE (scope, key), ON CONFLICT DO NOTHING RETURNING, an in_progress row written and committed before the processor call, and a stored response replayed on completion.
- Fire 50 concurrent identical requests with one key and assert one payment row, one processor call, and 50 byte-identical response bodies and status codes.
- Re-send the same key with amount_minor changed and assert 409 with the fingerprint-mismatch code and no new payment row.
- Kill the process between the processor call and the local commit, restart, and let the sweeper query the processor by the stored reference; assert the key converges to completed with exactly one charge.
- Advance the clock past expires_at, replay the key, and assert the documented behaviour (a new payment) rather than an undefined one.
Follow-up
- The processor call succeeds but your commit fails. What does the next request with the same key see, and what converges the row?
- A merchant reuses one key for two genuinely different payments a month apart. Which of your rules catches it, and which lets it through?
- Where does the key live end to end, and what changes if the downstream processor honours its own key?
Serve balances and statements without re-summing the ledger
Balances are derived from an append-only ledger_entry table: entry_id, transaction_id, account_id, direction (debit|credit), amount_minor, currency, source_type, source_id, business_date, posted_at. Reads run at 50,000/s from customer apps and merchant dashboards; postings run at 10,000 entries/s. Large accounts hold tens of millions of entries, so summing per read is not viable. Design the read path: where the current balance lives, when it is written relative to the entries, what a caller sees immediately after its own posting, and how a statement for a closed business date stays byte-identical on every future read. State your invalidation rule.
Approach
- Split the request into two reads with different requirements rather than one 'balance API'. Current balance is a single row that must be read-your-writes for the party who just posted. A statement is a bounded range over entries for one business_date window, and once that date's cutoff has passed the range is immutable, so it is cacheable indefinitely rather than for a guessed TTL.
- Write the materialised balance inside the same database transaction as the entries: UPDATE account_balance SET amount_minor = amount_minor + $delta, version = version + 1 WHERE account_id = $1, alongside the INSERTs. An asynchronous updater fed from the entry stream is the tempting alternative and is wrong here, because it makes the balance disagree with the system of record for exactly as long as the consumer lags, which is exactly when someone is refreshing the screen.
- Serve the balance as a primary-key lookup, O(1) and sub-millisecond, and serve the statement as an index range scan on (account_id, business_date, entry_id), O(k) in rows returned rather than O(n) in account history.
- Invalidate by writing through on commit, keyed by account_id and carrying the version, not by TTL. A TTL on a balance is a defined interval during which you knowingly display a stale number; a version lets a reader detect staleness instead of hoping.
- Price the design honestly: the extra balance UPDATE serialises postings on that account row, and that lock hold is what sets the per-account write ceiling. Measure it before deciding the read path is free.
Follow-up
- A customer disputes a balance shown three months ago. Reproduce that exact number from the data you kept.
- The materialised balance and the sum of entries differ by one minor unit. How do you find that before the customer does, and what do you do about it?
- What changes if the balance must also be readable from a second region with a 70 ms round trip?
Authorisation p99 tripled overnight with no deploy
Payment orchestration serves about 3,000 authorisations per second against a 150 ms p99 budget. Since 02:00, p99 is 460 ms and rising about 8 ms per hour, while p50 is unchanged at 11 ms. There was no deploy, no traffic change and no processor degradation. The hot path does one INSERT into idempotency_key, which has UNIQUE (scope, key), then two UPDATEs on that row: locked_at, then status and response_body. A nightly reconciliation job started at 01:50 and is still running. Give the ordered diagnostic checklist and the cause.
Approach
- Read the shape first. p50 flat with p99 rising and no deploy is a resource or data-volume effect, not a code path, because a code change moves the median too. A tail that climbs monotonically at fixed workload means something monotonically grows.
- Ask what started at 01:50. In PostgreSQL an open transaction holds back the xmin horizon cluster-wide, so autovacuum can reclaim no dead tuple newer than that snapshot. Confirm with pg_stat_activity (state, now() - xact_start, backend_xmin) and with pg_stat_all_tables (n_dead_tup, last_autovacuum) for idempotency_key.
- Connect it to the write pattern. Three writes per key produce up to two dead tuples each, so at 3,000 rps the table sheds roughly 6,000 dead tuples per second. Heap-only tuple updates would keep those out of the index, but only when no indexed column changes and the page has room, and appending response_body grows the tuple enough to force a new page. So the unique index on (scope, key) grows too.
- Explain why only the tail suffers. A larger index means more pages per lookup and a rising fraction of them missing shared_buffers; the median request still hits cache while the tail pays physical I/O. This is exactly the p50-flat, p99-rising signature, and it is worth stating before acting.
- Verify before fixing rather than after. n_dead_tup in the millions and rising, last_autovacuum stale since about 01:50, and pg_relation_size on the unique index measured twice fifteen minutes apart showing growth at constant workload. Index bloat cannot be inferred from row count alone; use the size series or pgstattuple.
- Fix in two moves and prevent separately. End or chunk the long transaction so the reconciliation job commits per batch instead of holding one snapshot over 50M lines, then let autovacuum catch up or run REINDEX CONCURRENTLY. Add a transaction-age alert and a statement timeout on the reporting role. Collapsing the two UPDATEs into one and lowering fillfactor halves dead-tuple production, but that is an optimisation, not the cause.
Follow-up
- The reconciliation job legitimately needs a consistent view of 50M lines. How do you give it one without pinning the xmin horizon?
- Why did p50 not move at all?
- You also run an expires_at cleanup job that DELETEs old idempotency keys. During this incident, does running it help or hurt, and what design avoids the question entirely?
For someone who has spent the last few years shipping features and reading other people's code, and who has not solved a timed problem from a blank file in a long time. Five days rebuild the primitives and the patterns that sit on them, working from invariants rather than remembered solutions, and the last two attach that back to the rest of the loop.
Prepare, practise & reflect
One practical outcome each day. Spend longer where you need it.
0 / 7 done01Rebuild the primitives by implementing them
- Implement a dynamic array with doubling growth and an operation counter, then change the growth rule to add a fixed sixteen slots instead, and time both for n of ten thousand, a hundred thousand and a million. The fixed-increment version resizes n/16 times at O(n) each, so its total work is quadratic; doubling is what makes append amortised constant.
- Implement a hash map with separate chaining and a load-factor resize, then insert ten thousand keys engineered to land in one bucket and record what happens to lookup time, so that average-case O(1) becomes a claim with a stated precondition rather than a reflex.
- For dynamic-array append and hash-map insert, write down which cost is amortised rather than worst-case, which single operation pays the whole bill, and what a system with a hard per-operation deadline would have to do instead.
Deliverable: Two working implementations plus a timing table showing the input at which each structure's advertised complexity stops holding.
Practice prompt ↗Practice prompt ↗Worked solution ↗02Arrays under an invariant: two pointers, sliding window, binary search
- Solve longest-subarray-with-sum-at-most-K using a sliding window, then run it on an input containing negative numbers and watch it return the wrong answer: extending the window only moves the sum monotonically when every element is non-negative, and that precondition is the whole reason the technique works.
- Write the binary search that finds the first index satisfying a predicate rather than an exact value, put the loop invariant above the loop in a comment, and verify termination on the two inputs that break careless versions: the empty range, and a range where every element satisfies the predicate.
- Compute the midpoint as lo + (hi - lo) / 2 and write one line on why the obvious (lo + hi) / 2 is a genuine defect in a fixed-width integer type and a non-issue in a language with arbitrary-precision integers.
Deliverable: Three solved problems, each with its invariant written above the loop, plus one recorded input on which the sliding window is provably wrong.
Practice prompt ↗Practice prompt ↗03Sorting, heaps, and the greedy argument that has to be proved
- Solve one top-k problem three ways, by full sort, by a size-k heap, and by quickselect, then write the values of n and k at which each becomes the right choice, along with quickselect's quadratic worst case and why a randomised pivot makes that unlikely rather than impossible.
- Implement bottom-up heapify and count sift-down steps to confirm it does linear work rather than n log n, because most nodes sit near the bottom of the tree and therefore move only a short distance.
- Take interval scheduling by earliest finishing time and write the exchange argument out in full: given any optimal schedule, swapping in the earliest-finishing interval keeps it feasible and no smaller. Then construct the weighted variant where that same greedy fails and name what has to replace it.
Deliverable: A three-way top-k comparison with measured crossover points, one written exchange argument, and one counterexample to a greedy rule that looks almost identical.
Practice prompt ↗Practice prompt ↗04Recursion, memoisation, and the step to a table
- Take one problem with overlapping subproblems, such as edit distance or coin change, instrument the plain recursion with a call counter to show the blow-up, then add memoisation and re-count.
- Convert the memoised version to a bottom-up table and state the two properties you relied on: each subproblem's result depends only on its arguments, and the dependencies form a DAG you can enumerate in order.
- Rewrite one deep recursion with an explicit stack, then find the input length at which the original hits the interpreter's frame limit, which defaults to about a thousand frames in CPython, so you know when the rewrite is required rather than decorative.
Deliverable: One problem in three forms, naive, memoised and tabulated, with call counts for each and the input length at which recursion depth becomes the binding constraint.
Practice prompt ↗Practice prompt ↗Worked solution ↗05Graphs, where most of the work is choosing the traversal
- Implement BFS and DFS over one adjacency list, then answer for each which finds a shortest path in an unweighted graph and which you would use to detect a cycle in a directed graph, including why the in-progress versus finished distinction matters for the second.
- Implement topological sort by in-degree, feed it a graph containing a cycle, and confirm the failure signature is that fewer than V nodes come out rather than an exception, then note that the order it produces is one of several valid ones.
- Run a shortest-path search on a graph with a single negative edge weight and show the wrong answer, then write the precondition Dijkstra actually needs, non-negative weights, because it finalises a node's distance the first time that node is popped, and name the algorithm you would switch to and its own limit.
Deliverable: A small graph library with BFS, DFS and topological sort, plus two inputs that produce documented wrong answers under the wrong algorithm choice.
Practice prompt ↗Practice prompt ↗06One day for everything that is not an algorithm
- Sketch one system only to the depth a coding-heavy loop tends to reach: the endpoints, what the service stores, and the single query pattern that decides the schema. Stop at twenty-five minutes.
- Prepare the project answer for an interviewer who codes, which means rehearsing the two levels they push to: the specific thing you built, and why you chose that approach over the alternative they will name. Open with a number and be ready to say what it excludes.
- Prepare the answer to what you would do differently, choosing a real technical mistake with a specific fix rather than a complaint about process or staffing.
Deliverable: One design sketch at endpoint-and-schema depth, plus a project answer rehearsed to two levels of follow-up.
Practice prompt ↗Practice prompt ↗07Solve out loud, under time
- Do three timed problems at twenty-five minutes each in a plain editor with no autocomplete and no execution until the end, then tally separately the failures that were syntax and the ones that were approach, because those two numbers call for different fixes.
- Narrate one solution from the first sentence, stating the approach and its complexity before writing any code, and rehearse the sentence you will use when you realise mid-solution that the approach is wrong.
- Re-solve from blank the two problems you were slowest on this week and compare the times against the day they first appeared.
Deliverable: A recording of one fully narrated solution and a tally that separates syntax failures from approach failures.
Practice prompt ↗Worked solution ↗Expand any day for tasks and deliverables. Your progress is saved on this device.
Engineers over-index on what they repaired. A stronger answer covers something you knowingly left broken: the alert you tuned down, the data inconsistency you documented instead of chasing, the cleanup you deferred past two quarters. Give the reasoning and the condition that would have reopened it, so it reads as a decision and not as neglect.
How do you handle situations where you have to explain technical trade…
How do you handle situations where you have to explain technical trade-offs to non-technical stakeholders?
Approach
- Name the disagreement and how you resolved it with evidence.
- Give the blast radius: what could have broken, and what you measured.
- Pick a story where you made the decision, not one where you watched it.
Follow-up
- How did you know your change caused the improvement?
- What did you decide not to do, and why?
Describe a time you had to pivot your technical approach due to changi…
Describe a time you had to pivot your technical approach due to changing requirements.
Approach
- Give the blast radius: what could have broken, and what you measured.
- Name the disagreement and how you resolved it with evidence.
- Pick a story where you made the decision, not one where you watched it.
Follow-up
- What would you do differently if you ran that again?
- How did you know your change caused the improvement?
Estimate a reconciliation rebuild you have never attempted
You are asked how long it takes to replace a reconciliation service matching 30 million settlement lines a day against the ledger, including a bounded fuzzy fallback for netted fees and an ageing model for breaks. You have never built one. Produce an estimate, the range around it, and the two or three unknowns that dominate that range. Then describe a time you estimated unfamiliar work: what you did in the first day to shrink the range, what you committed to publicly, how far off you were, and what you would tell the requester differently now.
Approach
- The probe is whether you can be useful under uncertainty without either refusing to estimate or inventing false precision. Give a number with an explicit range and the basis for both, then immediately name what would move it, rather than asking for two weeks of discovery first.
- Decompose into parts with different uncertainty profiles. The hash join on (external_reference, amount_minor, currency, business_date) over 30 million lines is well understood engineering and estimates tightly; the fuzzy fallback for netted and fee-adjusted lines does not, because its scope is defined by whatever the files actually contain; the ageing and break workflow is mostly operations-facing surface area, which estimates by counting screens and states.
- Name the dominating unknowns concretely: how many distinct file formats and cutoff conventions the sources use, what fraction of lines are netted rather than itemised, and whether business_date is derivable from any field in the file or must be reconstructed from the cutoff rule. Each is a factor on the fuzzy path, not a percentage on the whole.
- Describe the first-day range-shrinking work, which is the part that separates strong from generic: take one real file, count distinct formats, measure the netted fraction, and attempt the exact join on a single day of postings to see what the residual actually is. One day of that typically converts a 3x range into something near 1.5x.
- Commit in a form that survives being wrong: a range plus a checkpoint date at which you will replace it with a narrower one, and an explicit statement of what you will cut first if the range turns out to be optimistic.
- In the retrospective half, give the real numbers: the estimate, the actual, and the specific thing that consumed the difference. Answers that were within 10 percent are less informative than answers that were 2x off for a nameable reason.
Follow-up
- The requester wants one number, not a range, for a board deadline. What do you give them?
- Your one-day probe finds 40 percent netted lines instead of the 5 percent you assumed. What changes in the plan, not just the estimate?
- What do you cut first if you are at the deadline and the fuzzy fallback is not done?
- 01
How do you handle situations where you have to explain technical trade-offs to non-technical stakeholders?
- 02
Describe a time you had to pivot your technical approach due to changing requirements.
- 03
You are asked how long it takes to replace a reconciliation service matching 30 million settlement lines a day against the ledger, including a bounded fuzzy fallback for netted fees and an ageing model for breaks. You have never built one. Produce an estimate, the range around it, and the two or three unknowns that dominate that range. Then describe a time you estimated unfamiliar work: what you did in the first day to shrink the range, what you committed to publicly, how far off you were, and what you would tell the requester differently now.
Is this an official Opto Investments interview guide?
No. It is PracHub's own research and practice material for the Software Engineer role at Opto Investments. Rounds and questions reflect what candidates have reported, not a process Opto Investments has published, and they change over time. Confirm the current format and scope with your recruiter.
PracHub interview research ↗How much time should I dedicate to preparation?
Dedicate at least two to three weeks to review core computer science concepts, practice coding problems, and research the Opto Investments platform. Quality of preparation—focusing on edge cases and clean code—is more important than the volume of problems solved.
PracHub interview research ↗What differentiates a successful candidate?
Successful candidates are those who ask clarifying questions before jumping into code and who communicate their thought process clearly throughout the interview. They also demonstrate genuine curiosity about the financial problems Opto Investments solves.
PracHub interview research ↗Is the interview process mostly remote?
Yes, the Opto Investments interview process is primarily conducted online, using video conferencing and collaborative coding environments to simulate a real work setting.
PracHub interview research ↗Sources & methodology 3 sources ↗
No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.
- 01PracHub interview research ↗
PracHub editorial research into this company and role, maintained with this guide. Candidate-reported, not an employer publication.
PracHub page · Accessed 2026-09-22 - 02PracHub Software Engineer practice ↗
Cross-company practice questions for this role.
PracHub page · Accessed 2026-09-22 - 03PracHub interview preparation framework ↗
The framework the preparation plan follows.
PracHub page · Accessed 2026-09-22
Explore related engineering environments and compare the preparation focus.
Ramp Software Engineer Interview Guide
Systems practice for Ramp software engineer candidates, weighted toward payments and lending work: the load that actually arrives, the failure modes that matter there, and the invariants a partial outage must not break.
Plaid Software Engineer Interview Guide
API design practice for Plaid software engineer prep: which changes existing clients survive, how an optional field differs from a removed one, and what a contract costs once it is public.
Capital One Software Engineer Interview Guide
Code-reading and review practice for Capital One candidates: tracing unfamiliar control flow, finding the invariant a function quietly depends on, and separating a correctness defect from a matter of taste.
Brex Software Engineer Interview Guide
Timed practice for Brex software engineer candidates who lose time to the clock: short problems taken end to end, then reviewed against a written rubric rather than a passing test run.