Subarray Sums Divisible by K
A medium-tier problem at 56% community acceptance, tagged with Array, Hash Table, Prefix Sum. Reported in interviews at thoughtspot and 3 others.
Subarray Sums Divisible by K is a medium-difficulty problem that combines prefix sums with hash table logic to find subarrays meeting a modular constraint. You'll see it at Citadel, Expedia, Thoughtspot, and Tinkoff. The naive approach times out fast: you can't brute-force every subarray and check divisibility. The real trick is a two-liner insight about modular arithmetic and prefix sums that most candidates miss in the live assessment. If you blank on the pattern, StealthCoder surfaces a working solution invisible to the proctor.
Companies that ask "Subarray Sums Divisible by K"
Subarray Sums Divisible by K is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe key is recognizing that if two prefix sums have the same remainder when divided by K, the subarray between them is divisible by K. Most candidates see 'divisible' and jump to brute force or a sliding window that doesn't exist here. The actual solution uses a hash table to track prefix sum remainders as you iterate. When you see a remainder you've seen before, you've found valid subarrays. The pitfall: handling negative remainders and the edge case of remainder zero. StealthCoder is your hedge if the modular arithmetic trick doesn't click during the live OA and you're running out of time.
Pattern tags
You know the problem.
Make sure you actually pass it.
Subarray Sums Divisible by K recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Made by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Subarray Sums Divisible by K interview FAQ
Is this really a medium, or does it feel harder?+
It's genuinely medium once you see the prefix sum remainder pattern. The acceptance rate (55.6%) reflects that most candidates miss the insight and try brute force. The trick is small but non-obvious without drilling it.
Do I need to know modular arithmetic going in?+
Not formally. You need to understand that if prefix[i] mod K equals prefix[j] mod K, then the subarray between them sums to a multiple of K. That's the entire mathematical requirement. Most prep materials don't teach this connection clearly.
What's the main pitfall candidates hit?+
Trying to use a sliding window or two-pointer approach, which doesn't apply here. Another common trap: not initializing the hash table with remainder 0 to handle subarrays starting from index 0. Both kill your solution on the live assessment.
How does this relate to the other topics listed?+
Array and Hash Table are the tools. Prefix Sum is the algorithm. You iterate the array once, compute running sums modulo K, and store remainders in a hash table. All three topics are required for the correct solution.
Is this still asked at Citadel, Expedia, and the others?+
Yes. It appears in OA reports from all four listed companies. It's not trendy or new, it's a stable, medium-difficulty screening question that tests both pattern recognition and implementation hygiene.
Want the actual problem statement? View "Subarray Sums Divisible by K" on LeetCode →