Permutation Sequence
A hard-tier problem at 50% community acceptance, tagged with Math, Recursion. Reported in interviews at X and 4 others.
Permutation Sequence hits hard because it's asked at X, Jump Trading, Amazon, Adobe, and Google, yet most candidates blank on it cold. The problem looks like it's asking you to generate all n! permutations, find the kth one, and move on. But generating and searching through millions of permutations will TLE instantly. The trick is realizing you can compute which permutation you're in without building any of them. With a 50% acceptance rate, this is a real gatekeeper. If this lands in your live OA and you haven't drilled the factoradic number system pattern, StealthCoder runs invisibly and surfaces the solution in seconds.
Companies that ask "Permutation Sequence"
Permutation Sequence 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 an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too.
Get StealthCoderThe insight is that permutations follow a predictable mathematical structure. For any position in the sequence, you can calculate how many permutations start with each digit without generating them. This is where Math and Recursion meet. At each step, you figure out which digit should come next by dividing k by the factorial of remaining elements. Then you remove that digit and recurse on the smaller problem. Most candidates either try brute force, which fails on large n, or attempt backtracking without realizing the factorial math cuts the work to O(n^2). The common pitfall is treating k as 1-indexed when the math expects 0-indexed, or off-by-one errors in the factorial bounds. If you've seen factoradic representation before, this becomes mechanical. If not, StealthCoder hedges the live OA entirely.
Pattern tags
You know the problem.
Make sure you actually pass it.
Permutation Sequence 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 by an Amazon engineer who watched the leaked-problem repo become an industry secret. He decided you should have it too. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Permutation Sequence interview FAQ
Is Permutation Sequence still asked at big tech?+
Yes. It's reported by five major companies including Google, Amazon, and Jump Trading. At 50% acceptance, it's harder than the typical medium but not rare. It filters for candidates who understand combinatorial math, not just coding.
What's the actual trick to avoid TLE?+
Don't generate permutations. Instead, use factorial math to determine which digit comes next at each position. For n elements remaining and position k, the digit is at index k / (n-1)!. This runs in O(n^2) instead of O(n!).
How does this relate to Math and Recursion topics?+
Math: the factorial number system (factoradic) lets you map a position directly to a permutation. Recursion: after picking each digit, you recurse on the remaining digits and the adjusted k value. Both are essential to the solution.
What's the biggest gotcha candidates hit?+
Mixing 0-indexed and 1-indexed k. The problem gives k as 1-indexed, but the math expects 0-indexed. Convert to k - 1 at the start. Also, some candidates forget to remove used digits from the pool after each step.
How do I know if I'm ready for this in a real OA?+
You should be able to explain factoradic representation and trace through a small example (like n=3, k=2) without writing code. If you can't, the problem will ambush you. If you can but coding takes 20+ minutes, you're cutting it close.
Want the actual problem statement? View "Permutation Sequence" on LeetCode →