Microsoft·Data Scientist·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Jun 2026

Summary

Microsoft data scientist interview with a coding question that was more of an algorithms exercise than anything ML-related. One question, but they wanted the whole package: working code, complexity analysis, edge case tests, and proof the operation was in-place.

Questions Asked (1)

Write a function that reverses a Python list in-place, with O(n) time and O(1) extra space. No slicing, no built-in reverse helpers. Show it works for empty lists, single-element lists, and a specific six-element example, and include a test that confirms the list object id doesn't change.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The two-pointer swap is pretty standard but I spent an embarrassing amount of time second-guessing whether swapping with a temp variable counted as 'extra space.' It doesn't, obviously.

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AI HintsAI Generated

Suggested Approach

Use a two-pointer technique: initialize left at 0 and right at len(lst)-1, then swap elements while left < right, incrementing left and decrementing right. This achieves O(n) time and O(1) extra space. Then demonstrate with empty, single-element, and six-element lists, and verify in-place mutation by checking the list's id before and after.

Pro tip: Explicitly state that the function returns None to emphasize in-place modification, and mention that the id check confirms no new list object is created. Also, note that the algorithm handles edge cases naturally without special branching.

1. Clarify requirements and constraints

Restate the problem: reverse a list in-place, O(n) time, O(1) extra space, no slicing or built-in reverse. Confirm that the function should mutate the input list and return None.

2. Design the two-pointer algorithm

Explain the approach: use two indices, left starting at 0 and right at len(lst)-1. While left < right, swap lst[left] and lst[right], then move left forward and right backward.

3. Implement the function

Write the Python function with a clear name like reverse_in_place. Include a docstring specifying time and space complexity. Ensure no extra data structures are used.

4. Test edge cases and example

Test with an empty list, a single-element list, and a six-element list (e.g., [1,2,3,4,5,6]). Show that the function mutates the list correctly and returns None.

5. Verify in-place mutation

Capture the id of the list before and after calling the function, and assert they are equal. This proves the list object was not replaced.

Key Points to Mention

Two-pointer technique for O(n) time and O(1) space
In-place mutation: the list object identity (id) remains unchanged
Edge cases: empty list, single-element list, even and odd lengths
No slicing or built-in reverse functions used
Function returns None to signal in-place operation
Time complexity O(n) and space complexity O(1) explained

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