Number of Changing Keys
A easy-tier problem at 80% community acceptance, tagged with String. Reported in interviews at Autodesk and 0 others.
Number of Changing Keys is an easy string problem with a 79.7% acceptance rate, which means most candidates who attempt it pass. But it's asked by Autodesk, and that acceptance rate is a trap. The trick isn't algorithmic; it's in the careful counting of state transitions. You read a string, track when adjacent characters differ, and count those transitions. Sounds simple. Blank on the edge cases during your OA, and you'll waste 10 minutes debugging an off-by-one error. If this problem hits your live assessment and you freeze on the logic, StealthCoder surfaces a working solution invisibly, no proctor awareness.
Companies that ask "Number of Changing Keys"
Number of Changing Keys 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 Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe problem requires you to iterate through a string and count how many times the character changes from one position to the next. The naive approach works: compare each character to the previous one, increment a counter when they differ. The common failure point is handling the first character correctly (there's no transition before it) and ensuring you're counting transitions, not total characters. Some candidates overthink it and build a state machine or use regex when a single pass suffices. The real difficulty isn't the algorithm; it's precision under time pressure in a live OA. String problems can feel trivial until you're on camera and misread the specification. StealthCoder is the hedge for the one problem you didn't review before assessment day. It reads what's on your screen and delivers the exact pattern you need.
Pattern tags
You know the problem.
Make sure you actually pass it.
Number of Changing Keys recycles across companies for a reason. It's easy-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 Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Number of Changing Keys interview FAQ
Is this problem actually easy, or does the acceptance rate lie?+
The 79.7% acceptance rate is honest. The algorithm is straightforward: iterate and count state changes. The trap is implementation details. Off-by-one errors, mishandling the first character, or overthinking the loop logic cause failures. It's easy in concept, easy to mess up in execution under pressure.
What's the trick that makes candidates fail?+
No hidden trick. The common mistake is comparing the current character to itself instead of the previous one, or starting your counter at 1 instead of 0. Read the problem carefully: you're counting transitions between adjacent characters, not unique characters.
Do I need dynamic programming or advanced string techniques?+
No. A single forward pass through the string with a comparison variable is sufficient. This is a String topic problem, but it's testing careful iteration logic, not substring matching, pattern searching, or DP. Keep it simple.
Is Autodesk still asking this problem in their OAs?+
Autodesk is the only reported company in this problem's dataset, so visibility is limited. But it's an easy problem with high acceptance, so it's a good candidate for screening rounds. Don't skip it in prep; it's exactly the kind of problem that appears when you haven't studied.
How long should I spend on this in a real assessment?+
If you understand the pattern, 3 to 5 minutes to code and test. If you're second-guessing your loop logic or edge cases, you can waste 15 minutes. Know your boundaries before the OA. If you blank, StealthCoder runs invisibly and gives you the solution in seconds.
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