Trainline Software Engineer
Interview Questions & Guide 2026

def choose_offers(offers, currency, now):
    best = {}
    for o in offers:
        price = o["price_minor"]
        if type(price) is not int or price < 0:
            raise ValueError("nonnegative integer minor units required")
        if o["currency"] != currency or o["expires_at"] <= now:
            continue
        key = o["journey_id"]
        if key not in best or (price, o["offer_id"]) < (best[key]["price_minor"], best[key]["offer_id"]):
            best[key] = o
    return [best[k] for k in sorted(best)]

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CREATE TABLE carriers (id TEXT PRIMARY KEY);
CREATE TABLE tickets (id TEXT PRIMARY KEY, carrier TEXT, currency TEXT, minor INTEGER, status TEXT, at TEXT);
CREATE TABLE payment_attempts (ticket_id TEXT, attempt INTEGER);
INSERT INTO carriers VALUES ('c1'),('c2');
INSERT INTO tickets VALUES ('t1','c1','EUR',2500,'confirmed','2026-09-01T10:00:00Z'),
 ('t2','c1','GBP',900,'confirmed','2026-09-01T10:00:00Z'),
 ('t3','c2','EUR',800,'pending','2026-09-01T10:00:00Z');
INSERT INTO payment_attempts VALUES ('t1',1),('t1',2);
WITH totals AS (
 SELECT carrier,SUM(minor) AS value FROM tickets
 WHERE status='confirmed' AND currency='EUR' AND at<'2026-09-02T00:00:00Z'
 GROUP BY carrier
)
SELECT c.id,COALESCE(t.value,0) FROM carriers c
LEFT JOIN totals t ON t.carrier=c.id ORDER BY c.id;

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CREATE TABLE inventory (journey TEXT PRIMARY KEY, remaining INTEGER CHECK(remaining>=0));
INSERT INTO inventory VALUES ('j1',1);
UPDATE inventory SET remaining=remaining-1 WHERE journey='j1' AND remaining>0;
SELECT changes();
UPDATE inventory SET remaining=remaining-1 WHERE journey='j1' AND remaining>0;
SELECT changes();
SELECT remaining FROM inventory;

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