Minimum Window Substring

Problem: Find the minimum window in $s$ containing all characters of $t$.

Approach: Expand until window contains all required characters. Track with a frequency map and counter for satisfied character types. Once valid, shrink from left while maintaining validity, updating the answer.

Time: $O(|s| + |t|)$. Each character visited at most twice.

Space: $O(|s| + |t|)$ for frequency maps.

See the implementation unit for code.