Count of Range Sum
A hard-tier problem at 37% community acceptance, tagged with Array, Binary Search, Divide and Conquer. Reported in interviews at Morgan Stanley and 0 others.
Count of Range Sum is a HARD problem asking you to count subarrays whose sum falls within a given range. With a 37% acceptance rate, it's a real filter. Morgan Stanley has reported it. The trap is obvious: brute force checking all subarrays is O(n^2) or worse. You need to either merge sort, binary search trees, or segment trees to hit O(n log n) or O(n log^2 n). Most candidates blank on the data structure choice or the counting logic during the live OA. If this problem hits your assessment and you're not fluent in one of the tricks, StealthCoder surfaces a working solution invisible to the proctor.
Companies that ask "Count of Range Sum"
Count of Range Sum 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 engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe core insight: you can't just count pairs of prefix sums naively. You need to track how many previous prefix sums fall within [current_sum - upper, current_sum - lower] for each position. This is a range-query problem in disguise. Divide-and-conquer with merge sort works by sorting indices by value and maintaining an ordered structure (Binary Indexed Tree, Segment Tree, or Ordered Set) that answers 'how many values in range X to Y have been seen.' The merge sort approach is cleanest conceptually but tricky to code correctly. Binary Indexed Tree or Segment Tree is faster in practice. The failure mode: forgetting to handle negative ranges, off-by-one errors in the range query, or picking a data structure you don't know well. If you're solid on one method, drill it. If you hit this live and panic, StealthCoder has the pattern ready.
Pattern tags
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Count of Range Sum recycles across companies for a reason. It's hard-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 engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Count of Range Sum interview FAQ
Is this really asked at FAANG or big tech?+
Morgan Stanley has reported it. It's rare but not unheard of in quant or backend interviews. The 37% pass rate means most people don't solve it cleanly. It's more common in companies that care about algorithmic depth than typical product teams.
What's the trick I'm missing?+
Stop thinking subarrays. Think prefix sums and range queries. For each new prefix sum, you're asking 'how many old prefix sums fall in a specific range.' That reframe unlocks Divide and Conquer, Binary Indexed Tree, or Ordered Set. Pick one, master it, stop second-guessing.
Do I really need a Segment Tree or Binary Indexed Tree?+
No. Merge sort with Divide and Conquer works and is cleaner to code. Ordered Set (like TreeMap in Java) also works. Segment Tree and Binary Indexed Tree are faster in constant factors but harder to code bug-free under time pressure. Master one method, then optimize if you have time.
How does this relate to Merge Sort and Divide and Conquer?+
Merge sort naturally splits the problem: count ranges in the left half, right half, then count ranges that cross the midpoint. During the merge step, you query how many left-side prefix sums pair with each right-side prefix sum. It's a clean way to avoid O(n^2) brute force.
What mistakes kill most submissions?+
Off-by-one in the range bounds (lower/upper inclusive/exclusive confusion), forgetting that prefix sums can be negative, not initializing the prefix sum correctly, or picking a data structure you don't fully understand. Walk through a small negative-number example step by step before submitting.
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