Longest Common Prefix
A easy-tier problem at 45% community acceptance, tagged with String, Trie. Reported in interviews at CME Group and 48 others.
Longest Common Prefix shows up constantly at CME Group, HSBC, Yelp, and 46 other companies. It's marked easy, but the 45% acceptance rate tells you most candidates either brute-force it or miss edge cases under time pressure. The trick isn't algorithmic complexity. It's recognizing when horizontal scanning beats vertical, and knowing exactly which inputs will crater a naive solution. If this problem hits your live OA and you blank, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Longest Common Prefix"
Longest Common Prefix 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 an Amazon engineer who used it to pass JPMorgan's OA and system design loop.
Get StealthCoderThe naive approach compares character by character across all strings, which works but wastes operations. The real edge cases: empty input, single string, strings with no common prefix, or one string that's a prefix of another. A Trie is overkill for this problem and slower than it looks. The pattern most candidates miss is early termination. Stop comparing the moment you find a mismatch. Horizontal scanning (iterate through each string fully before moving to the next character) often outperforms vertical (character by character across all strings at once). Under assessment pressure, candidates freeze on the first approach that compiles and miss optimizations. StealthCoder hedges this by surfacing the pattern immediately, so you're not debugging implementation on the clock.
Pattern tags
You know the problem.
Make sure you actually pass it.
Longest Common Prefix 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 by an Amazon engineer who used it to pass JPMorgan's OA and system design loop. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Longest Common Prefix interview FAQ
Why is acceptance so low if it's marked easy?+
Edge cases wreck submissions. Empty arrays, null input, single string, or when strings have zero common prefix all trip up candidates who didn't test locally first. The logic is simple, but incomplete handling costs points under time pressure.
Is Trie actually the right approach here?+
No. Trie is correct but overkill and slower. Horizontal or vertical scanning with early termination beats it. Trie shines when you're searching many prefixes repeatedly. Here, you're finding one. Simpler wins.
Do CME Group and HSBC ask this the same way?+
Reported as asked at both, but exact formats vary by team and role. The core problem is stable: find the longest shared prefix across a list of strings. Implementation constraints differ. Ask the interviewer if there are performance requirements.
What's the trap in my first solution?+
Comparing all strings fully before checking the next character wastes time. Better approach: iterate one string, and for each character position, check if it exists at that position in every other string. Bail as soon as you hit a mismatch.
How does this relate to Trie as a topic?+
Trie is a secondary path. String matching is primary. Knowing String fundamentals (iteration, slicing, comparison) solves this cleanly. Trie is bonus knowledge, not required. Focus on String first.
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