Running Sum of 1d Array
A easy-tier problem at 87% community acceptance, tagged with Array, Prefix Sum. Reported in interviews at EPAM Systems and 1 others.
Running Sum of 1d Array is the canonical easy warm-up for prefix-sum problems, and it shows up in real OAs at EPAM Systems and TCS. With an 87% acceptance rate, this feels like a gimme until you're live and your brain short-circuits on the accumulator pattern. The problem is straightforward: transform an array into a running total. If you blank on the implementation during the assessment, StealthCoder surfaces a working solution in seconds, invisible to the proctor, so you can move on instead of burning 10 minutes on array indexing.
Companies that ask "Running Sum of 1d Array"
Running Sum of 1d Array 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. Built by a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe trap with this problem isn't the algorithm, it's the off-by-one error and deciding whether to mutate in place or build a new array. Most candidates know they need a running accumulator, but under timed pressure they mess up the loop bounds or return the wrong result. The core pattern is prefix sum: at each index i, store the sum of all elements from 0 to i inclusive. Some versions let you overwrite the input; others demand a new array. The observation that matters is that this is O(n) time and either O(1) or O(n) space depending on whether you're allowed to modify the input. If this problem hits your live OA and you blank on whether to use a separate result array or mutate the original, StealthCoder runs invisibly and gives you a correct, clean implementation instantly.
Pattern tags
You know the problem.
Make sure you actually pass it.
Running Sum of 1d Array recycles across companies for a reason. It's easy-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. Built by a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Running Sum of 1d Array interview FAQ
Why does this problem matter if it's easy?+
It's not about difficulty. Prefix sum is a foundational technique for harder array problems like subarray sums, product arrays, and range queries. Getting the pattern right here trains your instinct for cumulative computations. EPAM and TCS use it as a screening filter to spot careless mistakes under time pressure.
Can I do this in place or do I need a new array?+
The problem statement determines this. If you're allowed to mutate the input, you can do it in O(1) space by overwriting each element with its running sum. If not, build a result array. Read the constraint carefully in your actual assessment.
What's the actual trick here?+
There isn't one. That's the point. This problem tests whether you can translate the definition (each element becomes the sum of all prior elements plus itself) into clean loop logic without off-by-one errors. Most errors come from confusing the current element's role.
Is this still asked at EPAM and TCS?+
Yes. Both companies include it in their online assessments. It's a fast screening signal. You should nail it in under 2 minutes. If you don't, it signals carelessness, not lack of skill.
How does this relate to the Prefix Sum topic?+
This IS the prefix sum pattern in its purest form. Mastering it here makes range-sum and subarray problems much easier. Prefix sum is the technique; running sum is the simplest application of it.
Want the actual problem statement? View "Running Sum of 1d Array" on LeetCode →