St. Charles Trading Software Engineer Interview Guide

The 60-second brief

St. Charles Trading hires Software Engineers; this guide collects what candidates report about the process.

This guide is scoped to a Software Engineer candidate at St. Charles Trading.

Focus areas

• Object-Oriented Programming (OOP)
• Core OOP Concepts: Class
• Core OOP Concepts: Object

The St. Charles Trading interview process

St. Charles Trading candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

HR Screening Call

Initial call with HR to assess candidate's background and fit for the role.

What to demonstrate

• Initial call with HR to assess candidate's background and fit for the role
• Depth in Object-Oriented Programming (OOP)

How to prepare

• Be able to walk your CV end to end in two minutes, and say why this company specifically.
• Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Technical Panel Assessment

Panel interview focusing on technical capabilities and problem-solving skills.

What to demonstrate

• Panel interview focusing on technical capabilities and problem-solving skills
• Depth in Object-Oriented Programming (OOP)

How to prepare

• Answer aloud and timed: What is the difference between method overloading and method overriding?
• Answer aloud and timed: How does exception handling work, and when should you use a try-catch block?

Manager Discussion

Final discussion with a manager to evaluate team fit and collaborative culture.

What to demonstrate

• Final discussion with a manager to evaluate team fit and collaborative culture
• Depth in Object-Oriented Programming (OOP)

How to prepare

• Answer aloud and timed: What is an array, and how does it differ from a list or a hashmap in terms of memory and performance?
• Answer aloud and timed: Write a program to find and print all duplicate elements within a given string.

Where candidates lose points

Master Your Resume Details

Be prepared for a comprehensive review of your resume. You must be able to explain the architecture, technology choices, and business impact of every project and job experience you have listed.

Explain Your Thought Process

During technical discussions and coding challenges, communicate constantly. Interviewers value your logical approach, problem-solving methodology, and communication skills just as much as your final code.

A queue or buffer with no bound

Every producer-consumer boundary needs a capacity and a policy for reaching it: block the producer, shed load, or drop the oldest entry. Unbounded buffering converts a temporary slowdown into memory exhaustion and hides the backpressure signal that would have revealed the consumer was falling behind.

Questions & practice

How does exception handling work, and when should you use a try-catch block?

Medium · Object-Oriented Programming (OOP) & Fund

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

What is an array, and how does it differ from a list or a hashmap in terms of memory and performance?

Medium · Object-Oriented Programming (OOP) & Fund

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

Write a program to find and print all duplicate elements within a given string.

Medium · Coding & Data Structures

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. 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?

How would you implement a binary tree search, and what is its time complexity?

Medium · Coding & Data Structures

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. 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?

Given an array of integers, find two numbers that add up to a specific target sum.

Medium · Coding & Data Structures

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. 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?

Explain the difference in time complexity between searching an item in a List versus a HashMap.

Medium · Coding & Data Structures

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. 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?

How would you optimize a nested loop algorithm to run in linear time?

Medium · Coding & Data Structures

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. 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?

Describe your experience with asynchronous programming and how you handle multi-threading.

Medium · Systems, Web & Databases

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

How do you optimize a slow-running SQL query, and what is the role of database indexing?

Medium · Systems, Web & Databases

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

Stop tag and share joins from fanning out a page

Medium · join fan-out · semi-joins · many-to-many

resource_tag is (resource_id, tag_id) with PK (resource_id, tag_id); resource_share is (resource_id, shared_with_user_id, permission). The tagged-and-shared listing inner-joins resource to both, filters tenant_id, tag_id = ANY($2) and shared_with_user_id = $3, orders by updated_at DESC and takes 50. Pages come back with fewer than 50 distinct resources and the total in the header is far too high. Explain the row multiplication, rewrite both the page query and the count query so each is correct, and name the index each one needs. PostgreSQL 16.

Approach

1. Do the arithmetic against the predicates that are actually there. An inner join emits one row per matching child row, and both joins are filtered: tag_id = ANY($2) admits only the requested tags, shared_with_user_id = $3 admits one user's share rows. So a resource holding three of the requested tags and shared with $3 once yields three rows, not one — the multiplier is its count of matching tags times its share rows for that single user, and that second factor is 1 unless the table admits duplicate (resource_id, shared_with_user_id) pairs. LIMIT 50 then limits rows rather than resources, and COUNT(*) counts pairs — the header is the product, not the population.
2. Reject DISTINCT as the fix. It deduplicates after the product has been built, so the planner must materialise and sort the fanned-out set before the LIMIT can apply, and it leaves any SUM or AVG in the same select list wrong.
3. Rewrite both filters as semi-joins, keeping resource as the only row source: AND EXISTS (SELECT 1 FROM resource_tag rt WHERE rt.resource_id = r.resource_id AND rt.tag_id = ANY($2)) and the same shape against resource_share. A semi-join stops at the first match per resource and preserves the driving index order, so ORDER BY updated_at DESC, resource_id DESC LIMIT 50 still stops after 50 rows.
4. Count with the same predicates and no join at all: SELECT count(*) FROM resource r WHERE r.tenant_id = $1 AND r.status = 'active' AND EXISTS (...) AND EXISTS (...). Nothing multiplies a resource, so the number is the population.

Follow-up

• The filter changes from 'any of these tags' to 'all of these tags'. Rewrite it and state what it costs relative to the ANY form.
• A resource can be shared with the same user twice under different permissions. Does your count change, and should it?

What are the four pillars of Object-Oriented Programming, and how do you apply them in your code?

Medium · Object-Oriented Programming (OOP) & Fund

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

Explain the difference between a class and an object.

Medium · Object-Oriented Programming (OOP) & Fund

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

What is the difference between method overloading and method overriding?

Medium · Object-Oriented Programming (OOP) & Fund

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

What is a REST API, and what are the primary HTTP methods used to interact with it?

Medium · Systems, Web & Databases

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

Explain the difference between authentication and authorization in a secure web application.

Medium · Systems, Web & Databases

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

What happens behind the scenes when a user sends an HTTPS request to a server?

Medium · Systems, Web & Databases

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

Every query on one table stalls for forty seconds mid-deploy

Medium · lock contention · ddl · migrations

During a release on PostgreSQL, every query touching resource times out for about 40 seconds and then recovers with no intervention. The release ran one migration, ALTER TABLE resource ADD COLUMN archived_reason TEXT, and the migration log shows it completing in 6 ms. Unrelated tables showed no change in error rate. Explain how a 6 ms statement caused a 40-second stall, give the ordered checks you would run on a live system to confirm it, and give the migration procedure that prevents a repeat.

Approach

1. Separate the statement's duration from the lock's duration. ADD COLUMN with no default is a catalogue-only change and genuinely runs in milliseconds, but it requires ACCESS EXCLUSIVE, and it cannot acquire that until every transaction already touching the table has finished.
2. Account for the queueing, which is the part that surprises people. A lock request that is waiting blocks later requests for conflicting modes behind it rather than letting them overtake, so one long-open transaction holds the DDL and the DDL holds all the traffic. The stall length is set by the longest open transaction, not by the size of the change.
3. Confirm on a live system in this order: pg_stat_activity for that table ordered by xact_start, looking for the oldest transaction and specifically for state = idle in transaction; then pg_locks where granted = false to find the waiter; then join them on pid to name blocker and blocked. pg_blocking_pids() does that join for you and is the fastest single call.
4. Prevent rather than merely time it better. Set lock_timeout to a second or two on the migration session so the DDL abandons the queue after a bounded wait and is retried, instead of holding it for as long as the oldest transaction lives. Be exact about what that buys: queries arriving during the wait still queue behind the pending ACCESS EXCLUSIVE request, so each attempt costs them up to one lock_timeout of added latency. The outage goes from 40 seconds to about one second per attempt, not to zero. Also run migrations away from deploy-time peaks, and put a statement timeout and an idle-in-transaction timeout on the analytics role that opens the long transactions.

Follow-up

• The same release also wants NOT NULL on that column. What is the sequence that gets there without a long lock?
• Your lock_timeout retry fails ten times in a row because the analytics transaction is always open. What do you change?

Suggested practice checklist

Built from the rounds and topics St. Charles Trading candidates report.

Day 1 — Map the St. Charles Trading loop

• Write out the reported sequence: HR Screening Call, Technical Panel Assessment, Manager Discussion.
• For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

Day 2 — Work Object-Oriented Programming (OOP)

• Spend the session on Object-Oriented Programming (OOP), which St. Charles Trading candidates report being tested on.
• Write one worked example in Object-Oriented Programming (OOP) and time yourself on it.

Deliverable: One timed worked example in Object-Oriented Programming (OOP).

Day 3 — Work Core OOP Concepts: Class

• Spend the session on Core OOP Concepts: Class, which St. Charles Trading candidates report being tested on.
• Write one worked example in Core OOP Concepts: Class and time yourself on it.

Deliverable: One timed worked example in Core OOP Concepts: Class.

Day 4 — Work Core OOP Concepts: Object

• Spend the session on Core OOP Concepts: Object, which St. Charles Trading candidates report being tested on.
• Write one worked example in Core OOP Concepts: Object and time yourself on it.

Deliverable: One timed worked example in Core OOP Concepts: Object.

Day 5 — Answer out loud: Object-Oriented Programming (OOP) & Fundamentals

• Answer aloud, timed: What are the four pillars of Object-Oriented Programming, and how do you apply them in your code?
• Answer aloud, timed: Explain the difference between a class and an object.

Deliverable: Spoken answers to 2 reported Object-Oriented Programming (OOP) & Fundamentals question(s), under time.

Day 6 — Answer out loud: Coding & Data Structures

• Answer aloud, timed: Write a program to find and print all duplicate elements within a given string.
• Answer aloud, timed: How would you implement a binary tree search, and what is its time complexity?

Deliverable: Spoken answers to 2 reported Coding & Data Structures question(s), under time.

Day 7 — Answer out loud: Systems, Web & Databases

• Answer aloud, timed: What is a REST API, and what are the primary HTTP methods used to interact with it?
• Answer aloud, timed: Explain the difference between authentication and authorization in a secure web application.

Deliverable: Spoken answers to 2 reported Systems, Web & Databases question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Tell me about a time when project requirements changed mid-development and how you handled it.

Medium · Behavioral & Situational

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Describe a situation where you had a disagreement with a teammate and how you resolved it.

Medium · Behavioral & Situational

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Tell me about a time you had to step up and take a leadership role on a project.

Medium · Behavioral & Situational

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

How do you handle a situation where you are assigned a task using a technology you have never worked with befo

Medium · Behavioral & Situational

How do you handle a situation where you are assigned a task using a technology you have never worked with before?

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?