Custom Sort String
A medium-tier problem at 72% community acceptance, tagged with Hash Table, String, Sorting. Reported in interviews at Meta and 1 others.
Custom Sort String is the kind of problem that trips up candidates who think they need to build a custom comparator. Meta and Snap both ask it, and at a 72% acceptance rate, it looks easier than the pattern actually is. The gotcha is that you're not sorting the input string in the traditional sense. You're reordering one string based on the character frequency and ordering constraints defined by another string. Most candidates start building a comparator or trying to manipulate indices when the real solution is simpler. If this problem hits your live OA and you blank on the approach, StealthCoder solves it in seconds, invisible to the proctor.
Companies that ask "Custom Sort String"
Custom Sort 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. Made by a working FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know.
Get StealthCoderThe trick here is recognizing that you need to count character frequencies in the input string, then build the result by iterating through the sort string and appending each character as many times as it appears in the input. The naive mistake is trying to implement a full sort or comparator logic. What actually works: hash table to count frequencies, then a single pass through the sort string pulling characters from the hash table in that order. Characters in the input that aren't in the sort string get appended at the end, or omitted depending on the problem variant. This is fundamentally a counting and reconstruction problem, not a sorting problem. Hash tables and strings are the core topics here, and the sorting label is almost a red herring. When you hit this live, the math is trivial once you see the pattern. StealthCoder is the hedge for candidates who overshoot the complexity.
Pattern tags
You know the problem.
Make sure you actually pass it.
Custom Sort 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. Made by a working FAANG engineer who treats the OA the way companies treat hiring: as a game with rules you should know. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Custom Sort String interview FAQ
Is Custom Sort String actually hard?+
No. It's medium difficulty because the pattern isn't immediately obvious from the problem statement. Once you see that it's counting frequencies and then reconstructing based on sort order, it's straightforward. Most rejections happen because candidates try to implement a comparator or sort the string directly instead of doing the count-and-reconstruct approach.
Why do Meta and Snap ask this?+
It tests whether you can read a problem carefully and choose the right data structure. Hash tables for frequency counting are fundamental. It's not about algorithmic cleverness. It's about recognizing that you don't need a sort algorithm at all, which filters candidates who overthink.
What's the time complexity?+
O(m + n) where m is the length of the sort string and n is the length of the input string. You count frequencies once (n), then iterate through the sort string (m) and reconstruct. No actual sorting happens, so you avoid O(n log n) entirely.
What are the common mistakes?+
Trying to use a custom comparator or sorting function. Forgetting to handle characters in the input string that don't appear in the sort string. Off-by-one errors when appending characters. The hash table approach sidesteps most of these if you implement the count-and-reconstruct pattern cleanly.
How does this relate to the topics?+
Hash Table handles frequency counting. String is the data you're manipulating. Sorting is the label because the problem asks you to 'sort', but the actual algorithm doesn't use a sort. Understanding this mismatch is the key to solving it fast.
Want the actual problem statement? View "Custom Sort String" on LeetCode →