Check if Point Is Reachable
A hard-tier problem at 43% community acceptance, tagged with Math, Number Theory. Reported in interviews at PhonePe and 0 others.
You're looking at a hard-rated problem that sits at the intersection of number theory and geometry. PhonePe has asked it. The acceptance rate is 43 percent, which means nearly three out of five candidates who attempt it fail. Most people see a coordinate problem and immediately think BFS or DFS, then realize their solution times out or misses the actual insight. The real trick lives in number theory, not graph traversal. If this hits your live assessment and you're stuck on the spatial intuition, StealthCoder solves it in seconds while your screen stays locked down.
Companies that ask "Check if Point Is Reachable"
Check if Point Is Reachable 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 an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too.
Get StealthCoderThe problem disguises itself as a reachability question but it's actually a modular arithmetic problem. You're given a starting point and a target point, plus some movement rules. The naive approach is to simulate the movement or do a graph search, but the state space is infinite or huge. The actual pattern: apply the Euclidean algorithm or GCD logic to the movement deltas. Figure out what points are reachable using number theory constraints, not traversal. Most candidates waste 20 minutes on BFS before realizing the moves have a mathematical structure that either allows the target or forbids it entirely. When you understand the modular constraint, the answer is O(log n). StealthCoder is your safety net if the number-theoretic reduction isn't obvious during the timed session.
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You know the problem.
Make sure you actually pass it.
Check if Point Is Reachable 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. Made by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Check if Point Is Reachable interview FAQ
Is this actually a hard problem or is it 'hard' because it's mislabeled?+
It's legitimately hard, but not for the reason you think. The difficulty isn't in coding complexity. It's in recognizing that a geometry problem is really a number theory problem. Once you see the GCD or modular pattern, the implementation is short. Most candidates get stuck on the mental shift, not the code.
Should I try BFS or DFS first to understand the reachability?+
No. That's the trap. The state space is too large or infinite, so simulation fails. Start by thinking about what mathematical properties the movement operations have. Write down a few reachable and unreachable points, then look for the pattern. Number theory, not traversal.
PhonePe asked this. How brutal was it in their interviews?+
It's a hard-tier problem at PhonePe, so they used it to filter strong candidates. The 43 percent acceptance rate reflects that most people don't crack the number-theoretic insight under time pressure. Preparing specifically on Math and Number Theory topics helps.
What's the key insight I should look for?+
Look at the GCD of your movement deltas and how they constrain which points you can reach modulo some value. The reachability check often boils down to a single GCD computation or a check against a modular invariant, not a search.
How long does this usually take to solve in an interview?+
If you see the pattern, 10-15 minutes. If you don't, you'll burn 30-40 minutes on a dead-end simulation approach. The time sink is recognizing the pattern, not coding it. That's where StealthCoder buys you the minutes you need.
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