Num of Idle Robots 🐳
Reported by candidates from Amazon's online assessment. Pattern, common pitfall, and the honest play if you blank under the timer.
Amazon's April 2024 OA included a problem about tracking idle robots. You're likely looking at a simulation where robots move, perform tasks, or reach certain states, and you need to count how many end up idle. The trick isn't the counting, it's understanding the rules that determine when a robot becomes idle. StealthCoder will catch you if you misread the state transitions during the live assessment.
Pattern and pitfall
This is a simulation problem dressed as a counting problem. Robots follow rules (movement, task completion, energy depletion, or waiting for tasks), and your job is to track which ones hit an idle state by a deadline or condition. The common trap is mishandling concurrent states or forgetting edge cases like robots that never get idle or robots that idle immediately. You'll likely need to iterate through time steps, update robot states, and count at the end. If the robot rules are complex, you might simulate; if time is a factor, look for patterns or a closed-form answer. StealthCoder runs live and will flag if you're simulating inefficiently.
If you see this problem in your OA tomorrow, the play is to recognize the pattern in 30 seconds. StealthCoder buys you that recognition.
You can drill Num of Idle Robots 🐳 cold, or you can hedge it. StealthCoder runs invisibly during screen share and surfaces a working solution in under 2 seconds. The proctor sees the IDE. They don't see what's behind it. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken.
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Amazon reuses patterns across OAs. Built by an Amazon engineer who passed his OA cold and still thinks the filter is broken. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Num of Idle Robots 🐳 FAQ
What makes this problem hard?+
Not the counting. The hard part is correctly interpreting what 'idle' means in the problem context. It's usually not obvious from the title. Read the rules carefully: does idle mean no tasks assigned, no energy, waiting for something, or stationary for a period. Misread that and your logic is wrong.
Is this a brute-force simulation or do I need math?+
Start by simulating. Track each robot's state (position, task, energy, time idle) step by step. If the time constraint is huge (like 10^9 steps), look for periodicity or a formula. For most Amazon OAs, simulation with clean state management wins.
What's the trick with robot movement or tasks?+
Robots often act independently but may share resources (tasks, charging stations). Don't assume they're sequential. Model them in parallel if the problem allows simultaneous actions. Also watch for robots that block each other or compete for limited tasks.
How do I avoid off-by-one errors?+
Clarify the time window. Does idle count at time T, before T, or after T. Also clarify if a robot idle on turn 0 counts. Test your logic on a small example by hand before coding. One misalignment kills half your test cases.
Can I solve this in 48 hours with no prep?+
Yes, if you can simulate cleanly. Read the problem twice. Build a robot class or data structure. Write the state-update logic step by step. Test on the examples. If it passes those, you're likely correct. The pattern is simulation, not a hidden algorithm.