Longest Consecutive Sequence
A medium-tier problem at 47% community acceptance, tagged with Array, Hash Table, Union Find. Reported in interviews at Wissen Technology and 35 others.
Longest Consecutive Sequence lands in roughly half of all attempts, which means it's on the radar at Google, ByteDance, Walmart Labs, and Oracle. The trap is obvious: sort the array and scan for runs. That works, but it's O(n log n) and you'll watch the interviewer's face flatten when you say it. The real solution is O(n) and uses a hash table in a way that breaks the naive intuition. If this problem hits your live assessment and you blank on the pattern, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Longest Consecutive Sequence"
Longest Consecutive 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. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop.
Get StealthCoderThe trick is to build a hash set of all numbers, then iterate through once more. For each number, you only start counting a sequence if it's the beginning of one. That filters out 99 percent of the work. Most candidates either sort (wrong complexity), or iterate through the set multiple times per number (inefficient). Union Find also works but it's slower in practice here. The key insight is recognizing that you don't need to process every number as a potential sequence start. You skip ahead using set membership. When the assessment timer is running and the approach isn't clicking, StealthCoder hedges against blanking on the optimization.
Pattern tags
You know the problem.
Make sure you actually pass it.
Longest Consecutive Sequence 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. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Longest Consecutive Sequence interview FAQ
Why is sorting wrong for this problem?+
Sorting gives you O(n log n) time, but the optimal solution is O(n). Interviewers at Google and ByteDance will accept sort as a first pass, but they're checking if you can optimize. The hash table approach is the real bar at this difficulty level.
Is this still asked at big tech?+
Yes. It's reported at Google, Oracle, ByteDance, and ServiceNow among the top companies. The 47 percent acceptance rate shows it's medium-hard, not a gimme. It's common enough that it's worth drilling the O(n) hash table logic.
How do you avoid iterating through every number multiple times?+
Check if each number is the start of a sequence before you count. If num-1 exists in the set, skip it. This cuts the inner loop work from O(n) to O(1) per sequence. That's where the linear time comes from.
When would you use Union Find instead of a hash table?+
Union Find works but slower in practice for this problem because merge operations carry overhead. Hash table is cleaner and faster. Save Union Find for problems where you're actively grouping disjoint sets or checking connectivity.
What's the main pitfall candidates hit?+
Overthinking it. You don't need Union Find, and you don't need to sort. Build a set, iterate once, and count runs starting from actual sequence starts. Simplicity is the whole point. Candidates often add complexity that isn't there.
Want the actual problem statement? View "Longest Consecutive Sequence" on LeetCode →