K-th Smallest in Lexicographical Order
A hard-tier problem at 46% community acceptance, tagged with Trie. Reported in interviews at Hulu and 3 others.
K-th Smallest in Lexicographical Order is a Trie problem that most candidates get wrong the first time because the obvious brute-force approach times out immediately. You can't just generate all numbers and sort them. Hulu, DE Shaw, Microsoft, and Samsung have all asked this, and it shows up in screens where the interviewer is testing whether you can optimize beyond the naive path. If you haven't internalized the pattern, and it lands in your live OA, StealthCoder will surface a working solution in seconds while staying invisible to the proctor.
Companies that ask "K-th Smallest in Lexicographical Order"
K-th Smallest in Lexicographical Order 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 for the engineer who has done the work but might still blank with a webcam pointed at him.
Get StealthCoderThe trick is building a conceptual Trie of all numbers from 1 to n (represented as strings) without materializing it. You navigate the tree by counting how many numbers live in each subtree, then skip entire branches to land on the k-th element. Most candidates try to generate candidates in lexicographical order and stop at k, which works for tiny n but explodes past 10^6. The gap is thinking about the problem as a tree traversal where you calculate subtree sizes on the fly. Once you know a subtree has fewer than k remaining numbers, you skip it entirely. This approach scales to n in the billions. StealthCoder handles the off-by-one errors and the edge cases around digit boundaries that trip up implementations under time pressure.
Pattern tags
You know the problem.
Make sure you actually pass it.
K-th Smallest in Lexicographical Order recycles across companies for a reason. It's hard-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 for the engineer who has done the work but might still blank with a webcam pointed at him. Works on HackerRank, CodeSignal, CoderPad, and Karat.
K-th Smallest in Lexicographical Order interview FAQ
What's the actual trick to this problem?+
Stop generating numbers. Instead, treat 1 to n as a Trie of lexicographic strings. For each prefix, count how many valid numbers fall under it by calculating subtree sizes. Navigate left or right based on whether k falls into the current subtree. This lets you find the k-th number without enumerating candidates.
Why does brute-force fail?+
Sorting or storing all n numbers is O(n log n) space and time. For n up to 10^10, that's impossible. The subtree-counting approach is O(log(n) squared) because you make at most log(n) moves, and each move calculates subtree sizes in O(log n) operations.
Is this actually asked at big tech companies?+
Yes. Hulu, DE Shaw, Microsoft, and Samsung have all reported this problem. It's a hard-level Trie problem that filters for candidates who can reason about tree structures without building them explicitly.
How does lexicographical order relate to Trie navigation?+
A Trie naturally orders strings lexicographically by structure. Each node's children represent the next valid digit. Traversing left to right gives lexicographical order. This problem exploits that by skipping subtrees entirely rather than exploring them.
What's the acceptance rate and why is it so low?+
It hovers around 46 percent, which is typical for hard problems that punish brute-force thinking. Most candidates either time out on generation or get stuck on the counting logic. The subtree-size trick is non-obvious without specific Trie experience.
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