Zeta | SDE 2 | 2025

My first round at Zeta for the SDE 2 role began with an introduction and a brief discussion about my current project. Following that, the interviewer presented a specific Data Structures and Algorithms problem. The task involved determining the number of ways to select three particles (electrons 'e' and protons 'p') from a given array to form a stable atom, maintaining index order. A stable atom was defined as one with no two consecutive identical particles and exactly three total particles.

Initially, I discussed a recursive approach. During our discussion, the interviewer hinted that a non-recursive solution was possible, so I then proposed a simpler approach using a for loop.

The first follow-up asked to calculate all consecutive and non-consecutive ways. I first offered an O(n^3) solution, then optimized it to O(n^2), and wrote the corresponding code.

For the second follow-up, the challenge was to achieve an O(n) solution. I provided an approach using hashmaps and counters, which utilized O(n) extra space.

We ran out of time after this. As we wrapped up, the interviewer mentioned an O(n) time and O(1) space solution that I hadn't come up with. Given this, I'm not expecting to receive a call for further rounds.