Find Unique Binary String
A medium-tier problem at 79% community acceptance, tagged with Array, Hash Table, String. Reported in interviews at Accenture and 0 others.
Find Unique Binary String is a medium-difficulty problem that sounds deceptive in its simplicity. You're given an array of n unique binary strings, each of length n, and you need to find one binary string of the same length that isn't in the array. The 70% acceptance rate tells you most people get it, but the catch is that a brute-force approach can tank your time complexity. Accenture has asked this one. If you hit it cold in an assessment and your first instinct is to generate all 2^n possibilities, StealthCoder surfaces the smarter pattern in seconds while your proctor sees nothing.
Companies that ask "Find Unique Binary String"
Find Unique Binary 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. Built by a senior engineer who knows the OA is theater. This is the script.
Get StealthCoderThe trap most candidates fall into is trying to enumerate every possible binary string until finding a missing one. That works but wastes time and space. The actual trick involves treating this like a set membership problem: iterate through the input array, and for each string, derive a constraint that eliminates it from possibility. Backtracking and Hash Table strategies shine here because you can prune the search space intelligently. Array and String manipulation help you track which candidates remain. The elegant solution recognizes that with n strings of length n in a space of 2^n total strings, one is guaranteed to exist and can be found far faster than brute force. Understanding when to abandon exhaustive search and apply constraint propagation is what separates a correct answer from a correct answer that wastes 40% of your time.
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Find Unique Binary 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. Built by a senior engineer who knows the OA is theater. This is the script. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Find Unique Binary String interview FAQ
Is brute force really that slow for this problem?+
Yes. If n is even 20, generating 2^20 strings is over a million operations. The elegant approach avoids that. Since only n strings are forbidden out of 2^n possible, a smarter search strategy that eliminates impossibilities per string beats enumeration.
How does backtracking fit into the solution?+
Backtracking lets you build candidate strings digit by digit, pruning branches that can't lead to a solution. As you build, you check whether partial matches exist in the input. If not, that branch is safe. This keeps you from fully generating invalid strings.
Why use a Hash Table here?+
A Hash Table stores the input strings for O(1) lookup. Without it, checking membership is O(n) per candidate string. Hash Table lookup ensures your overall complexity stays polynomial instead of exponential.
Will companies still ask this after seeing it on LeetCode?+
Accenture has asked it. The problem tests whether you recognize the difference between brute force and constraint-based search. That pattern applies to many problems, so the core skill stays relevant even if the exact problem is known.
What's the most common mistake candidates make?+
Trying to enumerate all 2^n binary strings. Candidates often code this correctly but time out. The real test is realizing the set membership angle and using Hash Table to avoid redundant lookups during your search.
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