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This is a medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Microsoft.

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This question is commonly asked for Software Engineer candidates at Microsoft during technical interviews.

Sort Three Categories In Place | Microsoft Coding Question

Quick Overview

This question evaluates proficiency in in-place array manipulation, constant-space algorithms, and linear-time partitioning techniques. Commonly asked in Coding & Algorithms interviews to assess algorithmic efficiency, pointer/index management, and reasoning about invariants, it focuses on practical application of algorithmic concepts rather than purely theoretical understanding.

Quick Overview

Sort Three Categories In Place

Company: Microsoft

Role: Software Engineer

Category: Coding & Algorithms

Difficulty: medium

Interview Round: Technical Screen

You are given an array `items` containing only the values `0`, `1`, and `2`, where each value represents a category. Rearrange the array in-place so that all `0`s come first, followed by all `1`s, then all `2`s. Requirements: - Modify the input array in-place. - Use `O(1)` extra space. - Aim for a single-pass `O(n)` solution. Example: ```text Input: items = [2, 0, 2, 1, 1, 0] Output: items = [0, 0, 1, 1, 2, 2] ```

Quick Answer: This question evaluates proficiency in in-place array manipulation, constant-space algorithms, and linear-time partitioning techniques. Commonly asked in Coding & Algorithms interviews to assess algorithmic efficiency, pointer/index management, and reasoning about invariants, it focuses on practical application of algorithmic concepts rather than purely theoretical understanding.

You are given an array `items` containing only the values `0`, `1`, and `2`, where each value represents a category. Rearrange the array in-place so that all `0`s come first, followed by all `1`s, then all `2`s. You must modify the input array in-place, use `O(1)` extra space, and aim for a single-pass `O(n)` solution. For testing, return the modified array after sorting.

Constraints

0 <= len(items) <= 10^5
Each element in items is one of: 0, 1, 2
The solution must use O(1) extra space

Examples

Input: ([2, 0, 2, 1, 1, 0],)

Expected Output: [0, 0, 1, 1, 2, 2]

Explanation: After rearranging in-place, all 0s come first, then 1s, then 2s.

Input: ([0],)

Expected Output: [0]

Explanation: A single-element array is already sorted.

Input: ([],)

Expected Output: []

Explanation: An empty array remains empty.

Input: ([2, 2, 2, 1, 1, 0, 0],)

Expected Output: [0, 0, 1, 1, 2, 2, 2]

Explanation: The array contains all three categories in reverse-grouped order and must be rearranged in-place.

Input: ([1, 1, 1, 1],)

Expected Output: [1, 1, 1, 1]

Explanation: If all elements are the same category, the array does not change.

Input: ([2, 0, 1],)

Expected Output: [0, 1, 2]

Explanation: A small mixed example that requires swapping both ends correctly.

Hints

Try maintaining three regions in the array: one for `0`s, one for unknown values, and one for `2`s.
When you see a `2`, swap it toward the end, but do not immediately move past the new value swapped into the current position.

Last updated: May 15, 2026

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