Microsoft·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

Apr 2026

Summary

Microsoft SWE coding round, one classic sliding window problem. Nothing fancy, but the kind of question that humbles you if you haven't touched it in a while.

Questions Asked (1)

Find the longest substring that contains no repeating characters.

Algorithms & Data Structures

Author's notes

Knew this problem but still fumbled the implementation a bit.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem (e.g., input string, character set, expected output) and then present a sliding window solution using a hash map to track the last seen index of each character. Explain how the window expands and contracts to maintain uniqueness, and analyze time and space complexity.

Pro tip: Mention that the sliding window approach can be optimized to O(n) time by storing the last seen index of each character and jumping the left pointer directly, avoiding the need to shrink the window one step at a time.

1. Clarify the problem

Ask about input constraints (e.g., ASCII vs Unicode, empty string, case sensitivity) and expected output (length vs substring). Confirm that the substring must be contiguous.

2. Discuss brute force

Briefly mention the naive O(n^3) or O(n^2) approach of checking all substrings, but note it's inefficient for large inputs.

3. Propose sliding window

Explain the optimal O(n) approach using two pointers (left and right) and a hash map to store the last index of each character. Expand right, and when a duplicate is found, move left to max(left, lastIndex+1).

4. Walk through an example

Trace the algorithm on a sample string like 'abcabcbb' to demonstrate how the window and max length are updated.

5. Analyze complexity and edge cases

State time O(n) and space O(min(n, m)) where m is the character set size. Discuss edge cases like empty string, all unique characters, and all same characters.

Key Points to Mention

Sliding window technique with two pointers
Hash map to track last seen index of characters
Time complexity O(n) and space complexity O(min(n, m))
Handling edge cases: empty string, single character, all unique, all duplicates
Optimization: jumping left pointer directly to lastIndex+1
Comparison with brute force approach

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