Lyft·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026

Summary

Lyft software engineer interview with a classic sliding window problem. Nothing too exotic but the implementation details matter more than you'd expect.

Questions Asked (1)

Given two strings s and t, find the smallest substring of s that contains every character in t, including duplicates. Return an empty string if none exists.

Algorithms & Data Structures

Author's notes

I knew the sliding window pattern going in but still fumbled the frequency map bookkeeping.

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AI HintsAI Generated

Suggested Approach

Use a sliding window with two pointers to expand and contract a window over s while tracking character counts from t. Maintain a 'formed' counter to know when the window contains all required characters, and record the minimum length window. This yields O(n) time and O(k) space where k is the number of distinct characters in t.

Pro tip: Clarify edge cases upfront (e.g., t longer than s, empty strings, Unicode) and mention that the algorithm handles duplicates by tracking required counts. Also, discuss how you'd test it with examples like s='ADOBECODEBANC', t='ABC' to show thoroughness.

1. Clarify requirements and edge cases

Confirm that the substring must contain all characters of t including duplicates, and that order doesn't matter. Discuss edge cases: empty strings, t longer than s, no valid substring, and character set (ASCII vs Unicode).

2. Choose sliding window approach

Explain that a brute-force check of all substrings is O(n^2) or worse, so a sliding window with two pointers gives O(n) time. Use a frequency map for t and a window frequency map.

3. Expand and contract window

Move the right pointer to include characters, updating the window map and a 'formed' counter when a character's count meets the required count. When all characters are satisfied, move the left pointer to shrink the window while maintaining validity, updating the minimum length and start index.

4. Track and return result

Keep track of the minimum window length and its starting index. After the loop, return the substring if found, else an empty string.

5. Analyze complexity and test

State time complexity O(|s| + |t|) and space O(|s| + |t|) or O(k) where k is distinct characters. Walk through a test case like s='ADOBECODEBANC', t='ABC' to verify.

Key Points to Mention

Sliding window technique with two pointers (left and right).
Frequency maps (hash maps or arrays) to track required and window character counts.
The 'formed' counter to efficiently check when all required characters are present.
Handling duplicates by comparing counts, not just presence.
Time complexity O(n) and space complexity O(k) where k is distinct characters in t.
Edge cases: empty strings, t longer than s, no valid substring, and character encoding.

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