Ways to Make a Fair Array
A medium-tier problem at 64% community acceptance, tagged with Array, Prefix Sum. Reported in interviews at Dunzo and 4 others.
Ways to Make a Fair Array is a medium-difficulty array problem that's appeared in recent OAs at Dunzo, Twilio, Nvidia, PhonePe, and DoorDash. The 64% acceptance rate signals it's not trivial, but it's also not a rare algorithmic beast. The problem hinges on recognizing when a prefix sum approach unlocks a linear solution that a naive loop would botch. If you hit this live and freeze on the pattern, StealthCoder surfaces the working solution invisibly during your screen share.
Companies that ask "Ways to Make a Fair Array"
Ways to Make a Fair 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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe problem asks you to count valid operations or configurations where the array becomes balanced in some way. The trick is understanding that you're not checking every possible state independently, which would be O(n^2) or worse. Instead, prefix sums let you precompute cumulative info and answer each query in O(1) or collapse the whole problem into a single pass. The common pitfall is iterating and recalculating sums repeatedly. Many candidates will code a correct but slow solution, hit time limits, and panic. The pattern is always the same: build your prefix array once, then use it to answer the fairness question at each position. StealthCoder handles this instantly if you blank on whether to use prefix vs suffix or how to frame the balance condition.
Pattern tags
You know the problem.
Make sure you actually pass it.
Ways to Make a Fair Array 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Ways to Make a Fair Array interview FAQ
Is this problem asking about rebalancing or counting existing states?+
The title says 'ways to make,' so you're counting operations or configurations that result in a fair array. It's not about modifying in place. The prefix sum pattern emerges because you need to check a balance condition at each position efficiently, not by rebuilding state each time.
Why is 64% acceptance rate considered medium difficulty?+
That rate means roughly one-third of attempts fail, usually due to time limits or off-by-one logic around the balance condition. The algorithm is straightforward once you see it, but the implementation detail of prefix sums trips up candidates who default to nested loops.
Will Nvidia or DoorDash still ask this, or is it dated?+
It's in the active rotation at five named companies, so it's current enough. Big-N companies cycle patterns frequently, but Array and Prefix Sum fundamentals never go away. Don't ignore it as outdated.
How long should I spend on this if I'm stuck?+
If you see Array and Prefix Sum in the tags and can't spot the balance condition within 5 minutes, move on. The insight is either quick or you'll waste 20 minutes. StealthCoder is your hedge here, surfacing the solution instantly if time runs short.
What's the common mistake people make?+
Iterating through the array and recomputing sums at each index instead of precomputing them. Or confusing left sum vs right sum logic and getting the balance condition backwards. Prefix sums eliminate both mistakes if you structure it correctly from the start.
Want the actual problem statement? View "Ways to Make a Fair Array" on LeetCode →