Set Mismatch
A easy-tier problem at 45% community acceptance, tagged with Array, Hash Table, Bit Manipulation. Reported in interviews at Grammarly and 1 others.
Set Mismatch is an easy problem that shows up in real assessments at Grammarly and Microsoft, yet only 45% of submissions pass. It's the kind of problem where you think you know what's coming, then realize the obvious approach has a trap. You're given an array that should contain numbers 1 to n, but one number is duplicated and one is missing. Find both. It sounds simple until you're live in the assessment and your first instinct either uses too much memory or misses an edge case. StealthCoder solves it in seconds if you blank on the pattern.
Companies that ask "Set Mismatch"
Set Mismatch 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 trap here is thinking you need a hash table or a second array to track what you've seen. You do not. This problem is specifically designed to test whether you can solve it under tight constraints: Array, Hash Table, Bit Manipulation, or Sorting. Most candidates waste time hashing when the Array or Bit Manipulation approach gives you O(1) space. The key insight: use the array itself as a marker. Iterate through, and for each number x, flip the sign at index x-1 to mark that x has been seen. The number whose index was never flipped is missing. The number you try to flip twice is the duplicate. Sorting works too but costs O(n log n) time. During a live OA, if the constraint isn't explicit, the interviewer often hints at the clever solution. StealthCoder surfaces the in-place Array trick instantly so you're not stuck choosing between hashing and overthinking.
Pattern tags
You know the problem.
Make sure you actually pass it.
Set Mismatch recycles across companies for a reason. It's easy-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.
Set Mismatch interview FAQ
Why does this problem only have a 45% pass rate if it's marked easy?+
Candidates often misread the constraint or pick the wrong approach. Hash table works but signals you didn't think about space. The intended solution uses the array itself as a lookup table without extra memory. The pass rate reflects that many solve it, but fewer do it elegantly.
Is Set Mismatch still asked at Microsoft and Grammarly?+
Yes. Both companies have reported asking it. It's a screening-level problem that separates candidates who can optimize under constraints from those who default to the first working solution. It's not a gimmick question; it's core to their interview filter.
What's the trick that most candidates miss?+
Using the array indices as a hash table. Mark elements by negating the value at each index. When you hit an already-negative index, that's your duplicate. The one index that never gets negated is the missing number.
How does Bit Manipulation relate to this problem?+
You can solve it using XOR properties: XOR all input values and XOR all expected values 1 to n, then the result gives you the XOR of the duplicate and missing. It's elegant but harder to extend to finding both numbers. Most candidates prefer the Array approach.
Will I get time to think through this during the real OA?+
Typically yes, but the clock matters. If you're unsure, hash table gets you past it quickly. The in-place trick requires a moment to visualize. That's where StealthCoder becomes your safety net: invisible, instant, no time wasted second-guessing.
Want the actual problem statement? View "Set Mismatch" on LeetCode →