N26 Software Engineer
Interview Questions & Guide 2026

from collections import deque

class RollingTotal:
    def __init__(self, window):
        if type(window) is not int or window <= 0:
            raise ValueError("positive integer window required")
        self.window, self.events, self.total = window, deque(), 0
        self.last = None

    def add(self, now, amount):
        if type(now) is not int or type(amount) is not int:
            raise ValueError("integer timestamp and minor units required")
        if self.last is not None and now < self.last:
            raise ValueError("events must be ordered")
        while self.events and self.events[0][0] <= now - self.window:
            self.total -= self.events.popleft()[1]
        self.events.append((now, amount))
        self.total += amount
        self.last = now
        return self.total

r = RollingTotal(10)
assert [r.add(t, a) for t, a in [(0,100),(5,-20),(10,7)]] == [100,80,-13]

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CREATE TABLE accounts (id TEXT PRIMARY KEY, currency TEXT);
CREATE TABLE movements (account_id TEXT, amount_minor INTEGER, status TEXT, at TEXT);
INSERT INTO accounts VALUES ('a','EUR'),('b','GBP');
INSERT INTO movements VALUES
 ('a',100,'posted','2026-09-01T00:00:00Z'),
 ('a',-30,'posted','2026-09-01T12:00:00Z'),
 ('a',500,'pending','2026-09-01T13:00:00Z'),
 ('b',900,'posted','2026-09-02T00:00:00Z');
WITH totals AS (
 SELECT account_id, SUM(amount_minor) AS net
 FROM movements
 WHERE status='posted' AND at >= '2026-09-01T00:00:00Z'
   AND at < '2026-09-02T00:00:00Z'
 GROUP BY account_id
)
SELECT a.id, a.currency, COALESCE(t.net,0)
FROM accounts a LEFT JOIN totals t ON t.account_id=a.id
ORDER BY a.id;

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CREATE TABLE balances (account_id TEXT PRIMARY KEY, minor INTEGER NOT NULL);
INSERT INTO balances VALUES ('a',100);
UPDATE balances SET minor=minor-80
WHERE account_id='a' AND minor>=80;
SELECT changes();
UPDATE balances SET minor=minor-80
WHERE account_id='a' AND minor>=80;
SELECT changes();
SELECT minor FROM balances WHERE account_id='a';

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