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Meta·Machine Learning Engineer·Technical Phone Screen·Senior

SeniorPrefer not to say
May 2026Remote

Summary

Meta MLE coding round, one question the whole time. It was algorithmic, not ML-specific at all, which threw me a bit. The edge cases are where it gets painful.

Questions Asked (1)

Q1

Implement next permutation in-place: rearrange an array into its lexicographically next greater permutation, or reset it to ascending order if no greater permutation exists. Constant extra memory only.

Algorithms & Data Structures
Author's notes

I knew the algorithm in theory but fumbled the edge cases badly.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem and edge cases, then explain the three-step algorithm: find the pivot, find the successor, and reverse the suffix. Emphasize that the algorithm works in-place with O(1) extra space and O(n) time, and walk through an example to demonstrate correctness.

Pro tip: Mention that this algorithm is the same as the one used in C++'s std::next_permutation, and that it's a common interview question at top tech companies. Also, explicitly state the time and space complexity before writing code to show you think about efficiency.

1. Clarify and Edge Cases

Confirm the problem details: in-place, constant extra memory, and what to do if no next permutation exists. Discuss edge cases like empty array, single element, and already descending array.

2. Find the Pivot

Scan from right to left to find the first index i where nums[i] < nums[i+1]. If no such index exists, the array is in descending order, so reverse the entire array to get the smallest permutation.

3. Find the Successor

From the right, find the first element nums[j] that is greater than nums[i]. Swap nums[i] and nums[j].

4. Reverse the Suffix

Reverse the subarray from i+1 to the end to get the smallest possible suffix, ensuring the next permutation is the immediate next in lexicographic order.

5. Analyze Complexity and Test

State that the algorithm runs in O(n) time and O(1) space. Walk through a small example to verify correctness and discuss potential pitfalls.

Key Points to Mention

  • The algorithm consists of three main steps: find pivot, find successor, reverse suffix.
  • Time complexity is O(n) because each step involves at most one pass through the array.
  • Space complexity is O(1) because all operations are done in-place with only a few variables.
  • The pivot is the first element from the right that is smaller than its right neighbor.
  • If no pivot exists, the array is in descending order, so reversing it yields the smallest permutation.
  • The suffix after the pivot is in non-increasing order, so reversing it makes it non-decreasing, which is the smallest arrangement.

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