Hive.ai·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Coding round at Hive.ai for a software engineer role, one algorithmic problem that looks deceptively simple but has a few gotchas if you haven't seen it before.

Questions Asked (1)

Given an array of integers, sort it using only pancake flips. A pancake flip reverses the subarray from index 0 to k-1. Return the sequence of k values used. Any solution within 10 * n flips is acceptable.

Algorithms & Data Structures

Author's notes

I knew the general idea pretty quickly: find the max in the unsorted portion, flip it to the front, then flip it into its correct position at the end.

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AI HintsAI Generated

Suggested Approach

Use a selection-sort-like strategy: for each position from the end down to 1, find the maximum element in the unsorted prefix, flip it to the front, then flip it to its correct position. This guarantees at most 2n flips, well within the 10n limit. Return the list of flip sizes (k values) used.

Pro tip: Mention that the algorithm is optimal in the worst case (2n flips) and that you can optimize by skipping flips when the maximum is already at the front or at its correct position, reducing the constant factor.

1. Understand the problem and constraints

Clarify that a pancake flip reverses the prefix of length k, and the goal is to sort the array using at most 10n flips. Note that any sequence of flips that sorts the array is acceptable.

2. Choose a sorting strategy

Adopt a selection-sort approach: repeatedly place the largest remaining element at the end of the unsorted portion. This leverages the fact that flips can move any element to the front and then to any position.

3. Design the flip sequence

For each unsorted prefix of size m (from n down to 2), find the index of the maximum element. If it's not already at the end, flip it to the front (k = index+1), then flip the entire prefix (k = m) to move it to the end.

4. Implement and test

Write code to perform the flips, record each k, and verify the array is sorted. Test with edge cases like already sorted, reverse sorted, and arrays with duplicates.

5. Analyze complexity and optimize

Explain that the algorithm uses at most 2n flips (O(n) flips) and O(n^2) time for finding the maximum each time. Mention that you can skip unnecessary flips when the maximum is already in place.

Key Points to Mention

The algorithm guarantees at most 2n flips, which is within the 10n limit.
Each flip is O(1) to record, and the overall time complexity is O(n^2) due to scanning for the maximum.
The approach is similar to selection sort but uses prefix reversals instead of swaps.
Edge cases: empty array, single element, already sorted, reverse sorted, and duplicates.
Optimization: skip the first flip if the maximum is already at the front, and skip both if it's already at the correct position.
The solution is not required to be optimal in number of flips, but the 2n bound is simple and effective.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.