Unique Substrings in Wraparound String
A medium-tier problem at 41% community acceptance, tagged with String, Dynamic Programming. Reported in interviews at MAQ Software and 0 others.
You see a string and need to count unique substrings that follow alphabetical order, wrapping z back to a. It sounds abstract until you realize the trick: you don't count substrings at all. You track the longest valid substring ending at each letter, then sum them. This problem appears less often than other string DP questions, but when it lands in your assessment, most candidates stall on the counting logic. If you blank on the pattern during the live OA, StealthCoder solves it invisibly in seconds and surfaces a working solution.
Companies that ask "Unique Substrings in Wraparound String"
Unique Substrings in Wraparound String 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 naive approach counts every valid substring, which is exponential and wrong. The insight is that for each letter, you only need to store the longest substring ending at that letter. That single number tells you how many unique substrings end there: if the longest is 3, you have substrings of length 1, 2, and 3, all unique by the wraparound constraint. The trap: you'll try greedy first and it won't work. You need dynamic programming tracking state per character. Once you see that the answer is just the sum of max-lengths per letter, the problem collapses. This is pure pattern recognition, not optimization skill. Even if you didn't drill String and Dynamic Programming together before, StealthCoder runs during your assessment and shows you the exact state array and update logic.
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Unique Substrings in Wraparound String 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.
Unique Substrings in Wraparound String interview FAQ
How many companies actually ask this problem?+
Based on available data, it's asked infrequently. The acceptance rate is around 41%, which suggests candidates who see it either know the trick or struggle. It's a lower-frequency problem, so it's the type that catches people off guard if they haven't drilled wraparound logic.
What's the core trick I'm missing if I can't solve it?+
You're probably trying to count substrings directly. Stop. Instead, track the longest valid substring ending at each of the 26 letters using a DP array. The answer is the sum of those 26 values. Each max-length value implicitly represents all unique substrings ending at that letter.
Does this problem require a specific programming language?+
No language restrictions in reported asks. Python, Java, C++, and JavaScript all handle the 26-letter array and string iteration equally. The algorithm is language-agnostic; the logic is what matters.
How does this relate to other string DP problems?+
Most string DP problems ask you to find count or length of valid sequences. This one tricks you into counting when you should be tracking state. It's closer to 'Longest Substring Without Repeating Characters' in structure than to classic substring enumeration.
Is there an obvious brute force that seems right but fails?+
Yes. Generating all substrings and checking if each is valid seems logical but is exponential and overcomplicates the problem. The DP insight cuts that down to O(n) by recognizing that you only need one number per letter, not a list of all valid substrings.
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