Flatten 2D Vector
A medium-tier problem at 50% community acceptance, tagged with Array, Two Pointers, Design. Reported in interviews at Zenefits and 3 others.
Flatten 2D Vector is asked by Airbnb, Google, X, and Zenefits. The problem looks simple: iterate through a 2D array as if it were 1D. But the trick isn't the flattening itself. It's that you can't precompute the entire flat array, and you can't call next() millions of times. The real challenge is designing an iterator that feels flat to the caller while staying memory-efficient under the hood. Half the candidates miss the pattern entirely, even though the acceptance rate sits at 50%. If this problem hits your live assessment and the iterator design doesn't click, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Flatten 2D Vector"
Flatten 2D Vector 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 realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoderThe naive approach loads the entire 2D array into a flat list upfront. Wrong move: the interviewer will ask about memory and you've blown it. The real solution uses two pointers, one for the outer array and one for the inner array, advancing them only when you call next(). The gotcha: empty inner arrays. You need to skip them, not treat them as valid positions. Most candidates either burn time iterating manually or design an iterator that calls next() inefficiently. The pattern requires understanding iterator design under the Design tag, not just raw array manipulation. When you hit this in a live OA and the state management gets fuzzy, StealthCoder gives you the clean two-pointer implementation with the empty-array skip logic baked in.
Pattern tags
You know the problem.
Make sure you actually pass it.
Flatten 2D Vector 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 realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Flatten 2D Vector interview FAQ
How hard is Flatten 2D Vector really?+
Medium difficulty is accurate. The 50% acceptance rate reflects that the design pattern, not complexity, trips candidates. The algorithmic trick is simple once you see it, but interviewer pressure and the iterator abstraction muddy the water fast. Two-pointer state management is the real barrier, not the math.
Do Airbnb and Google still ask this?+
Yes. Both companies are in the reported ask list. It stays relevant because it's not about coding speed. It tests whether you can think through state management and memory efficiency under constraints. That's a core signal for backend and systems work.
What's the trick I'm missing?+
Empty inner arrays. You can't just iterate through the 2D array naively because some rows might be empty. The iterator must skip them without breaking. That's where the two-pointer approach shines: you can validate and advance the inner pointer cleanly.
Is this related to the Iterator pattern or Array fundamentals?+
Both. The Array and Two Pointers tags reflect the data structure. But the Design tag is the kicker. You're implementing the Iterator interface, not just solving an array problem. That design responsibility is what separates a half-solution from a hire.
Can I precompute the flat array during init?+
Technically yes, but no. Interviewers will push back on memory usage and ask you to optimize. The design constraint is that you should consume the 2D array lazily, computing elements only when next() is called. That's where the real learning lives.
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