Count Lattice Points Inside a Circle
A medium-tier problem at 55% community acceptance, tagged with Array, Hash Table, Math. Reported in interviews at Rubrik and 0 others.
You're given a radius and need to count how many integer coordinate points fall strictly inside a circle centered at the origin. Rubrik has asked this. It's a medium-difficulty geometry problem that looks deceptively simple until you hit the precision trap. The naive approach sounds easy: check every point in a bounding box against the distance formula. But floating-point arithmetic will trip you. The real trick is avoiding that trap entirely and working cleanly with integers. If you blank on the mathematical optimization during your live assessment, StealthCoder surfaces the working solution invisibly, so you don't lose time chasing false leads.
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Count Lattice Points Inside a Circle 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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know.
Get StealthCoderThe core insight is that you don't need to check all points in the bounding box naively. For each x-coordinate from -radius to radius, calculate the maximum y-coordinate where the point strictly satisfies x^2 + y^2 < radius^2. This reduces the problem from O(radius^2) comparisons to O(radius) iterations. The key is working entirely with integers and avoiding distance calculations altogether. Many candidates jump to floating-point distance checks, which creates subtle rounding errors and makes the boundary case ambiguous. Instead, rearrange the inequality algebraically: find the largest integer y such that y^2 < radius^2 - x^2. Then multiply by symmetry. The enumeration-based solution is clean and exact. This problem tests whether you can spot the optimization hiding inside what looks like a brute-force geometry problem. StealthCoder is your safety net if the pattern doesn't click before the timer starts.
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Count Lattice Points Inside a Circle 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 FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Count Lattice Points Inside a Circle interview FAQ
How do I handle the 'strictly inside' constraint without floating-point errors?+
Work entirely with squared distances and integer arithmetic. Check x*x + y*y < radius*radius, never compute sqrt or use floating-point comparisons. This avoids precision issues and makes the boundary exact. Rearrange to find the max y for each x algebraically.
Why doesn't the naive bounding-box approach work well here?+
A radius-sized bounding box has O(radius^2) points. Checking each one is slow and invites floating-point mistakes. Exploiting symmetry and iterating x once, then solving for max y, drops it to O(radius) and keeps you on integers throughout.
Is this problem still asked at companies like Rubrik?+
Yes, Rubrik has confirmed this problem. It appears in mid-level rounds. It's not a household favorite, but it shows up in geometry-heavy interview batteries. Expect it if the role touches spatial data or coordinate systems.
What's the acceptance rate and what does that tell me?+
Acceptance is around 55%, which is typical for medium-difficulty geometry problems. That means most candidates who see it don't solve it cleanly. The gap usually comes from floating-point bugs or missing the algebraic optimization entirely.
How does the Hash Table topic fit into this problem?+
Hash Table may appear if you use a set or map to memoize calculations or deduplicate counts for symmetry. It's not core to the solution, but some approaches cache computed y-values per x to avoid redundant math.
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