Count Collisions on a Road
A medium-tier problem at 44% community acceptance, tagged with String, Stack, Simulation. Reported in interviews at Arcesium and 0 others.
Count Collisions on a Road is a medium-difficulty string simulation problem with a 44% acceptance rate. Arcesium has asked it. The trap is thinking collision detection is hard. It's not. You'll read a string of characters (S, E, D for directions and speeds) and count how many cars crash into each other. The gotcha is figuring out which pairs actually meet, and the stack approach makes it elegant. If you hit this live and freeze on the collision logic, StealthCoder surfaces the working solution in seconds, invisible to the proctor.
Companies that ask "Count Collisions on a Road"
Count Collisions on a Road 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 naive approach is brute force: simulate every timestep and check distances. Slow, fiddly, and you'll off-by-one yourself into oblivion. The real pattern uses a stack. Left-moving cars (W) collide with right-moving cars (E) when they're adjacent or separated by stationary cars (S). Process right to left, push cars onto a stack, and when you see a direction change that means collision, count it and consume the cars. This is why Stack and String matter: the stack handles the collision chain elegantly without explicit time simulation. Most people miss that you don't simulate time at all. StealthCoder gives you the stack-based code when you're stuck mid-assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Count Collisions on a Road 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 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.
Count Collisions on a Road interview FAQ
Is this really just simulation, or is there a trick?+
There's a trick. Brute-force timestep simulation is slow and error-prone. The stack approach recognizes that collisions happen in a specific order based on direction changes and relative speed. Process the string smartly, not the timeline. That's what separates 44% from 100%.
Why do string and stack matter here?+
You're scanning a string left-to-right or right-to-left, and the stack tracks which cars are moving together or about to collide. The stack lets you handle chains of collisions without nested loops. It's a classic problem hiding under simulation clothes.
What's the most common wrong move?+
Overthinking time. Candidates try to simulate each second, checking distances, updating positions. You don't need that. Collisions are deterministic based on initial positions and directions. Stack the cars, match patterns, count collisions. Done.
Will Arcesium ask this again, or was it one-off?+
One company reported asking it, so treat it as a real possibility but not a common ask. That said, the pattern (stack-based collision logic on a string) shows up in other contexts. Worth knowing cold if you're interviewing there.
How much time do I need to solve this in an OA?+
If you know the stack trick, 15-20 minutes coded and tested. If you're simulating time, you'll burn 45 minutes debugging off-by-ones. The difference is knowing the pattern. If it appears mid-assessment and you blank, StealthCoder gives you the working code immediately.
Want the actual problem statement? View "Count Collisions on a Road" on LeetCode →