Smallest Missing Non-negative Integer After Operations
A medium-tier problem at 40% community acceptance, tagged with Array, Hash Table, Math. Reported in interviews at Mercari and 2 others.
You hit this problem on your Mercari, Atlassian, or IBM assessment and you've got 40 minutes left. The problem looks deceptively simple: find the smallest non-negative integer missing after you're allowed to perform operations on an array. But the twist is in the operations themselves, and most candidates misread what you're actually allowed to do. The greedy insight isn't obvious, and the hash table tracking gets messy fast. Only 40% of submissions pass. If you blank on the pattern mid-assessment, StealthCoder solves it invisibly while you stay calm and paste.
Companies that ask "Smallest Missing Non-negative Integer After Operations"
Smallest Missing Non-negative Integer After Operations 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 core trick here is recognizing that to find the MEX (minimal excludant), you need to greedily construct the smallest possible values starting from zero. Most people jump to simulation or brute force, which times out or misses edge cases. The winning approach uses a hash table to track which numbers you can make, then greedily tries to build 0, 1, 2, 3 in sequence. When you hit a number you can't construct, that's your answer. The math part is understanding which operations actually move you forward vs. trap you. Common pitfall: trying every operation combination instead of reasoning about the invariant you want to preserve. If this problem lands on your live assessment and the greedy structure doesn't click immediately, StealthCoder surfaces a working solution in seconds, giving you the confidence to explain it back.
Pattern tags
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Smallest Missing Non-negative Integer After Operations 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 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.
Smallest Missing Non-negative Integer After Operations interview FAQ
Is this problem really asking for MEX, and do I need to know that term?+
Yes, MEX (minimum excludant) is what the problem wants: the smallest non-negative integer not in your reachable set. You don't need the formal term, but understanding that you're building a contiguous range starting from zero is critical. The hash table stores which numbers you can produce.
Why does greedy work here instead of brute force?+
Greedy works because once you've confirmed you can make 0, 1, 2,..., k, the next number k+1 is either reachable (using one of the allowed operations) or it's not. If it's not, k+1 is your answer. Trying every operation combo is exponential and unnecessary. The operations constrain what's reachable in a way that greedy respects.
How do I track the hash table without getting lost?+
Start with the initial array values in your set. Then iterate through 0, 1, 2,... and for each candidate number, check if any allowed operation can produce it from numbers already in your set. If yes, add it. If no, return that number. This avoids duplicate work and keeps the logic linear.
What's the real pitfall that drops the acceptance rate to 40%?+
Misinterpreting which operations are allowed or what 'after operations' means. Some candidates waste time trying all orderings of operations instead of reasoning about what's mathematically reachable. Others forget to include the initial array values in their set. Edge case: the answer can be zero if the initial array doesn't contain it and you can't generate it.
Is this still asked at Mercari, Atlassian, and IBM?+
Yes, it appears in reports from all three. Medium difficulty, 40% pass rate, and a greedy-plus-hash-table pattern make it a good filter. It's not a rare unicorn problem. If you see it, the companies asking want to check your math reasoning and greedy intuition, not just your coding speed.
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