Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Jun 2026

Summary

Microsoft SWE coding round, one question the whole time. Pretty standard algorithmic problem but the in-place and single-pass constraints are where they actually want to see if you know what you're doing.

Questions Asked (1)

Given an array containing only 0s, 1s, and 2s, sort it in-place so all 0s come first, then 1s, then 2s. You must do it in a single pass with constant extra space.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I knew the three-pointer approach going in, but I fumbled explaining the loop invariant while coding it.

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AI HintsAI Generated

Suggested Approach

Use the Dutch National Flag algorithm with three pointers: low, mid, and high. Iterate through the array with mid, swapping elements to maintain three regions: 0s before low, 1s between low and mid, and 2s after high. This sorts the array in a single pass with constant space.

Pro tip: Emphasize that this is a classic partitioning problem and that the algorithm is optimal in time and space. Mention that it's also known as the Dutch National Flag problem, showing familiarity with standard algorithmic patterns.

1. Clarify the problem and constraints

Restate the problem to ensure understanding: sort an array of 0s, 1s, and 2s in-place, single pass, constant space. Confirm that the array can be modified and that no extra data structures are allowed.

2. Choose the algorithm

Select the Dutch National Flag algorithm (three-pointer approach) as it meets all constraints. Explain that it partitions the array into three sections: 0s, 1s, and 2s.

3. Define pointers and invariants

Initialize low = 0, mid = 0, high = n-1. Maintain invariants: elements before low are 0, elements between low and mid are 1, elements after high are 2, and the unexplored region is between mid and high.

4. Iterate and swap

While mid <= high, inspect arr[mid]. If it's 0, swap with arr[low] and increment both low and mid. If it's 1, just increment mid. If it's 2, swap with arr[high] and decrement high (do not increment mid).

5. Analyze complexity and edge cases

State that time complexity is O(n) and space is O(1). Discuss edge cases like empty array, all same elements, or already sorted array.

Key Points to Mention

Dutch National Flag algorithm (three-pointer technique)
In-place sorting with constant extra space
Single pass through the array
Time complexity O(n), space complexity O(1)
Handling of edge cases (empty array, all 0s, all 1s, all 2s)
Stability is not required, but the algorithm is not stable

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