Substrings of Size Three with Distinct Characters
A easy-tier problem at 75% community acceptance, tagged with Hash Table, String, Sliding Window. Reported in interviews at Quora and 2 others.
You're in an online assessment for Quora, Visa, or Accenture and hit this problem. It looks deceptively simple: count all substrings of exactly size three where all three characters are different. Your first instinct might be a nested loop checking each window manually. That works, but the moment you start coding it that way, you're burning cognitive load on bookkeeping instead of nailing the pattern. The acceptance rate sits at 75%, which tells you most people pass it, but the ones who don't usually hit themselves with off-by-one errors or redundant hash table logic. If you blank on the sliding window approach during the live OA, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Substrings of Size Three with Distinct Characters"
Substrings of Size Three with Distinct Characters 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe trick is sliding window with a single pass. You maintain a window of size three and track which characters appear in it using a hash table or set. For each new character you push into the window, you check if all three are distinct. If they are, increment your count. If the window exceeds size three, pop the leftmost character. The pitfall is overthinking it: you don't need to store frequencies or validate backwards. You're just counting valid windows as you move forward. Most wrong attempts either duplicate the checking logic, reset the hash table unnecessarily, or mishandle the boundary conditions when the string length is less than three. This problem rewards clean, direct sliding window code. It's a confidence builder before harder window problems. StealthCoder is the hedge if you freeze on the window-management syntax during your live assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Substrings of Size Three with Distinct Characters 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Substrings of Size Three with Distinct Characters interview FAQ
Is this problem really just about sliding window?+
Yes. You iterate once through the string, maintain a three-character window, and count windows where all three characters are distinct. Hash Table and Counting are the supporting topics, not the main driver. The pattern is pure sliding window.
What's the acceptance rate really telling me?+
At 75%, this problem filters for careless mistakes more than algorithmic ignorance. People pass when they get the sliding window right and boundary conditions clean. The ones who fail usually mix up window size, duplicate counting, or reset their hash table at the wrong time.
Do I need to optimize space or time?+
No. Single pass is O(n) time, and your hash table size is capped at 26 letters, so O(1) space. The problem doesn't ask for tricks. It's testing whether you can implement a straightforward window cleanly without off-by-one errors.
Why do Quora, Visa, and Accenture ask this?+
It's a filtering question. It's easy enough to pass quickly if you know sliding window, but easy enough to fail if you code carelessly. In a 45-minute OA, this separates candidates who can implement basic patterns accurately from those who rush and introduce bugs.
What's the most common mistake?+
Checking if all three characters are distinct using nested loops or repeated hash lookups instead of just checking the set size. Also, starting the count before you have a full window of three, or not handling the case where the string length is less than three.
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