Sort Characters By Frequency
A medium-tier problem at 74% community acceptance, tagged with Hash Table, String, Sorting. Reported in interviews at Wipro and 2 others.
Sort Characters By Frequency is a medium-difficulty problem that combines hash tables and sorting, and it's been asked at Wipro, Nvidia, and TCS. Most candidates see it as straightforward: count characters, sort by frequency, done. But the trick lives in the implementation choice. Do you sort the entire frequency map (O(n log n) overhead) or bucket by frequency (O(n) linear time). The assessment will accept either, but one shows you understand when to optimize. If you blank on the bucket-sort insight during the live OA, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Sort Characters By Frequency"
Sort Characters By Frequency 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 naive path is to hash all characters, then sort the frequency map by counts. It works, acceptance rate is high because most solutions pass. The trap is efficiency: sorting adds unnecessary log factor when frequency is bounded by string length. The optimal path uses bucket sort: create an array indexed by frequency, place each character in its bucket, then iterate buckets in reverse. This runs in linear time. Most candidates don't think of buckets for strings; they default to sorting library calls. During the assessment, when you're torn between the two approaches or stuck debugging a sort comparator, StealthCoder runs invisibly and gives you the bucket-sort code or the simpler sort-based version, whichever you need to move forward and lock in points.
Pattern tags
You know the problem.
Make sure you actually pass it.
Sort Characters By Frequency 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 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.
Sort Characters By Frequency interview FAQ
Why is this problem medium, not easy, if the approach is just count and sort?+
Acceptance rate is 74%, not 90%, because the time-complexity optimization (bucket sort vs. sorting) separates polished solutions from passing ones. Interviewers at Nvidia and Wipro probe whether you recognize when sorting is overkill. The problem rewards understanding the constraint: frequency is bounded by string length, not arbitrary values.
Do I have to use a heap or priority queue?+
No. A heap is one valid path but often unnecessary. You can use a hash table plus sort, or hash table plus bucket sort. All three pass. The heap approach adds complexity without earning bonus points unless you optimize for space or streaming scenarios, which the problem doesn't require.
Is bucket sort the expected solution here?+
It's the theoretically optimal solution (O(n) time), but sorting-based solutions are accepted at similar rates. TCS and Wipro interviewers care more that you can explain your choice and code it cleanly than which path you pick. If you're under time pressure in the assessment, a correct sort-based solution is safer than a rushed bucket implementation.
What's the most common pitfall?+
Forgetting the reverse iteration when you do use buckets. You build buckets by frequency and then iterate from the highest-frequency bucket backward. Off-by-one errors on array size and forgetting to include all characters are also frequent. Hash table setup is usually solid; the mistake lives in the output construction.
Will this problem appear in every tech interview I do?+
Lower frequency than array-sorting or string-reversal classics, but it's a reliable follow-up at mid-level assessments once you nail basic sorting. If it hits your live OA at Wipro, Nvidia, or TCS and you haven't drilled the bucket insight, StealthCoder's invisibility means you can still solve it in real time without the proctor knowing you had help.
Want the actual problem statement? View "Sort Characters By Frequency" on LeetCode →