Minimum Window Substring

Algorithms
Hard
Stripe
88.5K views

Given two strings `s` and `t`, return the minimum window substring of `s` such that every character in `t` (including duplicates) is included in the window.

Why Interviewers Ask This

Stripe engineers value robustness and efficiency in handling high-volume transactional data streams. This problem tests your mastery of the sliding window technique, a critical pattern for processing real-time data without redundant computations. Interviewers specifically look for your ability to manage character frequency maps dynamically and optimize time complexity from brute force O(n*m) to linear O(n), ensuring solutions scale under Stripe's strict latency requirements.

How to Answer This Question

1. Clarify constraints: Immediately ask about character sets (ASCII vs Unicode) and empty inputs to define edge cases early. 2. Visualize the logic: Propose using two pointers, 'left' and 'right', to represent the current window boundaries on string s. 3. Initialize structures: Explain creating a hash map or frequency array to store required counts from string t. 4. Expand phase: Describe moving the 'right' pointer to include characters until all requirements from t are met within the window. 5. Contract phase: Detail how to shrink the window from the 'left' while maintaining validity to find the minimum length, updating the best result found so far. 6. Optimize: Emphasize tracking a counter for matched characters to avoid re-scanning the frequency map during contraction. 7. Code cleanly: Write modular code with clear variable names, explaining the O(n) time and O(k) space complexity where k is the unique character count.

Key Points to Cover

  • Demonstrating the ability to reduce time complexity from quadratic to linear using sliding window
  • Correctly handling duplicate characters in the target string t via frequency maps
  • Properly managing edge cases such as missing characters or empty inputs
  • Explaining the logic for shrinking the window without recalculating the entire map
  • Clearly articulating space complexity relative to the alphabet size

Sample Answer

To solve the Minimum Window Substring problem efficiently, I would implement a sliding window approach that runs in linear time. First, I'd validate the input strings; if either is null or empty, I'd return an empty stri…

Common Mistakes to Avoid

  • Using nested loops to check every substring, resulting in inefficient O(n^2) or worse performance
  • Failing to account for duplicate characters in t, leading to incorrect validation logic
  • Not updating the minimum window length correctly when the window shrinks
  • Ignoring edge cases where the target string t contains characters not present in s

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