Find Common Characters
A easy-tier problem at 75% community acceptance, tagged with Array, Hash Table, String. Reported in interviews at Tripadvisor and 0 others.
Find Common Characters is an easy problem that hits your OA without warning. You're given an array of strings and need to return all characters that appear in every single string. Acceptance rate sits at 74%, which sounds high until you realize candidates blank on the counting logic under live pressure. Tripadvisor asks it. The trick isn't complex, but the implementation details trip people up. If this problem shows up during your assessment and you freeze, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Find Common Characters"
Find Common 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 by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage.
Get StealthCoderThe naive approach scans strings and tries to intersect character sets, but that fails on duplicates. The actual pattern: count character frequencies in the first string, then iterate through each remaining string and reduce counts to the minimum seen across all strings. You're building a frequency map, not a set. Most candidates conflate this with set intersection and waste time backtracking. The implementation uses an array for lowercase letters or a hash map for general Unicode. Common pitfall: forgetting to reset counts between strings or returning characters with zero frequency. This is straightforward once you see it, but under timed pressure, the counting logic becomes a friction point. StealthCoder handles the edge cases and the output format so you don't second-guess yourself.
Pattern tags
You know the problem.
Make sure you actually pass it.
Find Common 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 by an engineer who got tired of watching his cohort grind for six months and still get filtered at the OA stage. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Find Common Characters interview FAQ
Is Find Common Characters still asked at tech companies?+
Yes. Tripadvisor reports it. It's an easy-tier problem favored in screens where interviewers want to filter candidates who can't translate a simple spec into clean code. Don't underestimate it because of the difficulty label.
What's the trick to Find Common Characters?+
Stop thinking in sets. It's a frequency-counting problem. You need the minimum frequency of each character across all input strings, then output each character that many times. Hash Table solves it in one pass once you see the pattern.
How does Find Common Characters relate to the Array and String topics?+
You're iterating through an array of strings and extracting character data from each one. Array handles the iteration, String handles character extraction and counting. Hash Table bridges both by storing frequency maps.
What breaks candidates on this problem?+
Forgetting that characters can repeat in the output. If 'a' appears twice in every string, it appears twice in the result. Also, returning the counts instead of the characters themselves. Read the expected output format carefully.
How much time should I spend on Find Common Characters in prep?+
15 to 20 minutes once you understand frequency counting. It's a calibration problem. If this takes you longer than 30 minutes live, something's off and it's time to step back or call for help rather than spiral.
Want the actual problem statement? View "Find Common Characters" on LeetCode →