Implement a function that reverses a Python list in-place without allocating another list (no slicing, no list(), no reversed()). Requirements: O(n) time, O(1) extra space, and it must work for empty lists and odd/even lengths. Provide the function, brief correctness reasoning, and state the time/space complexity. Include tests that prove in-place behavior (e.g., the object id of the list is unchanged) and handle edge cases like [], [1], and [1, 4, 8, 12, 16, 20] → [20, 16, 12, 8, 4, 1].

This question evaluates a candidate's ability to implement an in-place list reversal and the underlying competencies in array/list manipulation, index/pointer management, and analysis of space and time complexity.

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Coding & Algorithms questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master coding & algorithms interviews.

What difficulty level is this interview question?

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

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This question is commonly asked for Data Scientist candidates at Microsoft during technical interviews.

Reverse a list in-place | Microsoft Interview Question

Coding Task: Reverse a List In-Place (Python)

Context

You're implementing a utility function during a technical screen. The function must reverse a Python list in-place without allocating another list.

Requirements

Implement a function that reverses a Python list in-place.
Constraints:
1. No slicing (e.g., arr[::-1]), no list(), and no reversed().
2. Time complexity: O(n).
3. Extra space: O(1).
4. Must work for empty lists, and for odd/even-length lists.
Deliverables:
1. The function implementation.
2. Brief correctness reasoning.
3. Time and space complexity.
4. Tests that demonstrate in-place behavior (e.g., object id unchanged) and cover edge cases like [], [1], and [1, 4, 8, 12, 16, 20] → [20, 16, 12, 8, 4, 1].

Coding Task: Reverse a List In-Place (Python)

Context

You're implementing a utility function during a technical screen. The function must reverse a Python list in-place without allocating another list.

Requirements

Implement a function that reverses a Python list in-place.
Constraints:
1. No slicing (e.g., arr[::-1]), no list(), and no reversed().
2. Time complexity: O(n).
3. Extra space: O(1).
4. Must work for empty lists, and for odd/even-length lists.
Deliverables:
1. The function implementation.
2. Brief correctness reasoning.
3. Time and space complexity.
4. Tests that demonstrate in-place behavior (e.g., object id unchanged) and cover edge cases like [], [1], and [1, 4, 8, 12, 16, 20] → [20, 16, 12, 8, 4, 1].

Reverse a list in-place

Quick Overview

Coding Task: Reverse a List In-Place (Python)

Context

Requirements

Solution

Comments (0)

Reverse a list in-place

Quick Overview

Coding Task: Reverse a List In-Place (Python)

Context

Requirements

Solution

Comments (0)