Range Sum Query - Immutable
A easy-tier problem at 69% community acceptance, tagged with Array, Design, Prefix Sum. Reported in interviews at Palantir Technologies and 1 others.
Range Sum Query - Immutable is the warm-up problem that looks trivial but teaches you a pattern that scales to the hard version. It's been asked by Palantir Technologies and Meta, and 68% of submissions pass, meaning people either nail it or miss the optimization entirely. The trap is thinking a naive loop through the range is fine. It's not. This problem is your first look at prefix sums, a technique that compounds across interview rounds.
Companies that ask "Range Sum Query - Immutable"
Range Sum Query - Immutable 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 an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoderThe pattern is simple but critical: build a prefix sum array during initialization, then answer any range query in O(1) time. The naive approach recalculates the sum for every query call, which tanks performance if queries outnumber the array size. Once you see that prefix_sum[i] = prefix_sum[i-1] + arr[i], the answer becomes mechanical. The trick is recognizing that the initialization cost is paid once, and every query after that is instant. This is exactly the kind of space-time tradeoff that separates solid engineering from careless coding. If this problem appears in your assessment and you blank on the prefix sum pattern, StealthCoder surfaces the solution in seconds, letting you move to harder problems.
Pattern tags
You know the problem.
Make sure you actually pass it.
Range Sum Query - Immutable 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 an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Range Sum Query - Immutable interview FAQ
Is this problem actually easy or is the difficulty rating wrong?+
It's genuinely easy once you know prefix sums. The 68% acceptance rate reflects that many candidates either know the pattern cold or haven't seen it yet. If you've drilled prefix sum problems, this is a gimme. If not, it's a gotcha.
What's the actual trick to Range Sum Query - Immutable?+
Build a prefix sum array at construction time where each index stores the sum of all elements up to that point. Then any range query becomes subtraction of two prefix values. This shifts the cost from query time to initialization, which happens only once.
Will this still come up at Meta or Palantir?+
Yes. Both companies have historically asked it. It's used as a screening or warm-up problem to identify candidates who understand basic optimization. Nailing it quickly builds momentum for harder problems.
How does prefix sum relate to the harder range sum problems?+
This problem is the foundation. Hard variants add 2D arrays, mutable updates, or offline queries. Mastering the immutable 1D version means you'll recognize the pattern immediately when it appears in a harder guise.
What's the space-time tradeoff here?+
You trade O(n) space for a prefix sum array to get O(1) query time. Without it, queries are O(n). In an assessment with many queries, the prefix sum approach is non-negotiable. It's also a signal that you think about scalability.
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