Google·Software Engineer·Onsite - Coding / Algorithms·Intermediate

IntermediatePrefer not to say

Jul 2026

Summary

Google SWE coding round, one question the whole time, sliding window style problem that sounds straightforward but has enough edge cases to keep you honest.

Questions Asked (1)

Given two strings s and t, find the shortest contiguous substring of s that contains every character of t with at least the same frequency. Explain your approach, justify it, analyze time and space complexity, and write working code.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

Classic minimum window substring.

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AI HintsAI Generated

Suggested Approach

Use a sliding window (two-pointer) technique to find the minimum window in s that contains all characters of t with required frequencies. Maintain a frequency map for t and a window frequency map, expanding the right pointer until the window is valid, then contracting the left pointer to minimize the window while keeping it valid. Track the smallest valid window found.

Pro tip: Clarify edge cases upfront (e.g., t longer than s, characters not in s) and discuss how your solution handles them. Also, mention that the sliding window approach is optimal and commonly expected at Google, but be prepared to discuss alternative approaches like binary search with hashing if asked.

1. Understand the problem and edge cases

Restate the problem to ensure clarity: find the shortest contiguous substring of s that contains every character of t with at least the same frequency. Discuss edge cases such as empty strings, t longer than s, or characters in t not present in s.

2. Choose the sliding window approach

Explain that a sliding window with two pointers efficiently finds the minimum window by expanding and contracting the window while maintaining character counts. Justify why this is optimal compared to brute force.

3. Detail the algorithm steps

Describe: 1) Build a frequency map for t. 2) Initialize left=0, min_len=infinity, min_start=0, and a counter for how many characters of t are satisfied. 3) Expand right, update window counts, and when all characters are satisfied, contract left to minimize the window, updating min_len and min_start. 4) Repeat until right reaches end.

4. Analyze time and space complexity

State that time complexity is O(|s| + |t|) because each character is visited at most twice (by left and right pointers). Space complexity is O(|s| + |t|) or O(1) if using fixed-size arrays for ASCII, but generally O(k) where k is the number of distinct characters in t and s.

5. Write and test the code

Implement the algorithm in a clean, modular way, using appropriate data structures (e.g., hash map or array). Walk through a small example to demonstrate correctness and handle edge cases in code.

Key Points to Mention

Sliding window technique with two pointers (left and right) to expand and contract the window.
Frequency maps (or arrays) to track required characters and current window counts.
A counter to track how many characters of t are satisfied in the current window.
Time complexity O(|s| + |t|) and space complexity O(|s| + |t|) or O(1) for fixed alphabet.
Handling edge cases: t longer than s, characters in t not in s, empty strings.
Correctness argument: the window is always valid when contracting, and we track the minimum length.

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