Lyst Software Engineer
Interview Questions & Guide 2026

def cheapest_available(offers, currency="GBP"):
    best = {}
    for offer in offers:
        price = offer["price_minor"]
        if type(price) is not int or price < 0:
            raise ValueError("price must be non-negative integer minor units")
        if not offer["in_stock"] or offer["currency"] != currency:
            continue
        product = offer["product_id"]
        candidate = (price, offer["offer_id"])
        if product not in best or candidate < best[product]:
            best[product] = candidate
    return {product: offer_id
            for product, (_, offer_id) in sorted(best.items())}

Scroll sideways to view long lines.

CREATE TABLE campaigns(id TEXT PRIMARY KEY);
CREATE TABLE clicks(id TEXT PRIMARY KEY, campaign_id TEXT, happened_at TEXT);
CREATE TABLE orders(id TEXT PRIMARY KEY, campaign_id TEXT, happened_at TEXT,
                    status TEXT, revenue_minor INTEGER);
INSERT INTO campaigns VALUES ('a'),('b'),('c');
INSERT INTO clicks VALUES ('a1','a','2026-09-01T01:00:00Z'),
  ('a2','a','2026-09-01T02:00:00Z'),('a3','a','2026-09-01T03:00:00Z'),
  ('b1','b','2026-09-01T04:00:00Z'),('next','a','2026-09-02T00:00:00Z');
INSERT INTO orders VALUES ('o1','a','2026-09-01T02:00:00Z','completed',2000),
  ('o2','a','2026-09-01T03:00:00Z','completed',3000),
  ('cancel','a','2026-09-01T04:00:00Z','cancelled',9000),
  ('boundary','a','2026-09-02T00:00:00Z','completed',7000);

-- Solution query
WITH click_totals AS (
    SELECT campaign_id, COUNT(*) AS clicks
    FROM clicks
    WHERE happened_at >= '2026-09-01T00:00:00Z'
      AND happened_at <  '2026-09-02T00:00:00Z'
    GROUP BY campaign_id
), order_totals AS (
    SELECT campaign_id, COUNT(*) AS orders,
           SUM(revenue_minor) AS revenue_minor
    FROM orders
    WHERE status = 'completed'
      AND happened_at >= '2026-09-01T00:00:00Z'
      AND happened_at <  '2026-09-02T00:00:00Z'
    GROUP BY campaign_id
)
SELECT c.id,
       COALESCE(ct.clicks, 0) AS clicks,
       COALESCE(ot.orders, 0) AS orders,
       COALESCE(ot.revenue_minor, 0) AS revenue_minor
FROM campaigns AS c
LEFT JOIN click_totals AS ct ON ct.campaign_id = c.id
LEFT JOIN order_totals AS ot ON ot.campaign_id = c.id
ORDER BY c.id;

Scroll sideways to view long lines.

def publish_snapshot(current_version, candidate_version, valid):
    if not valid:
        return current_version, "rejected-invalid"
    if current_version is not None and candidate_version <= current_version:
        return current_version, "ignored-stale"
    return candidate_version, "published"

Scroll sideways to view long lines.

def record_conversion(db, source, event_id, campaign_id, revenue_minor):
    with db:
        inserted = db.execute(
            "INSERT OR IGNORE INTO conversions VALUES (?, ?, ?, ?)",
            (source, event_id, campaign_id, revenue_minor),
        ).rowcount
        if not inserted:
            existing = db.execute(
                "SELECT campaign_id, revenue_minor FROM conversions "
                "WHERE source = ? AND event_id = ?",
                (source, event_id),
            ).fetchone()
            if existing != (campaign_id, revenue_minor):
                raise ValueError("event ID reused with different values")
            return False
        db.execute(
            "INSERT INTO revenue_totals VALUES (?, ?) "
            "ON CONFLICT(campaign_id) DO UPDATE SET "
            "revenue_minor = revenue_totals.revenue_minor + excluded.revenue_minor",
            (campaign_id, revenue_minor),
        )
        return True


import sqlite3

db = sqlite3.connect(":memory:")
db.executescript("""
CREATE TABLE conversions(source TEXT, event_id TEXT, campaign_id TEXT,
  revenue_minor INTEGER CHECK(revenue_minor >= 0), PRIMARY KEY(source,event_id));
CREATE TABLE revenue_totals(campaign_id TEXT PRIMARY KEY, revenue_minor INTEGER);
""")
assert record_conversion(db, "partner-a", "event-1", "campaign-a", 2500)
assert not record_conversion(db, "partner-a", "event-1", "campaign-a", 2500)
assert db.execute("SELECT revenue_minor FROM revenue_totals").fetchall() == [(2500,)]
db.close()

Scroll sideways to view long lines.