Last Substring in Lexicographical Order
A hard-tier problem at 35% community acceptance, tagged with Two Pointers, String. Reported in interviews at Fastenal and 1 others.
Last Substring in Lexicographical Order is a hard string problem with a 35% acceptance rate, asked by Fastenal and MathWorks. You need to find the lexicographically largest substring without generating every possible substring, which is the trap most candidates fall into. The trick involves two pointers and understanding how to skip comparisons when you know a substring can't possibly win. If this problem hits your live OA and the brute force approach times out, StealthCoder surfaces the optimal pattern instantly, invisible to the proctor.
Companies that ask "Last Substring in Lexicographical Order"
Last Substring 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. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe naive approach generates all substrings and sorts them, which fails on large inputs. The real solution uses two pointers to track candidate positions and a third pointer for comparison depth. When you find a mismatch during character-by-character comparison, you can eliminate entire ranges of substrings in one move because of lexicographical ordering. The key insight is that if substring starting at position A is smaller than the one at position B at some character offset, every substring starting between A and B up to that offset is also eliminated. This reduces what looks like O(n^2) or worse into a linear traversal. Two Pointers is the vehicle for this optimization. StealthCoder is your hedge if the pattern doesn't click during the real assessment.
Pattern tags
You know the problem.
Make sure you actually pass it.
Last Substring 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. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Last Substring in Lexicographical Order interview FAQ
Why does generating all substrings and sorting fail?+
Generating all n^2 substrings and sorting them is O(n^2 log n) time and O(n^2) space. On inputs over a few thousand characters, you'll hit memory and time limits. The problem requires you to find the answer without materializing all substrings.
Is this still asked at Fastenal and MathWorks?+
Yes. Both companies have reported asking this problem. It's a rare hard string problem, so if you see it in their OA, it's likely a deliberate filter for strong string and algorithmic intuition.
What's the two-pointer trick?+
Track two candidate start positions and compare characters at increasing offsets. When you find a mismatch, you can skip entire ranges of inferior substrings based on lexicographical properties. A third pointer marks comparison depth and eliminates candidates in bulk.
How does Two Pointers help here versus other string problems?+
Two Pointers usually means left and right converging. Here, both pointers move forward but represent competing substring candidates. The pattern is less common, which is why most candidates don't see it until they read the solution.
What if I blank on the pattern during the OA?+
A brute force solution might pass small test cases but will time out on hidden large inputs. If you hit a wall, StealthCoder runs invisibly and gives you the optimized two-pointer approach in seconds, letting you copy a working solution and move on.
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