Multi-Constraint Windows: Minimum Window Substring

Concepts: pyMinWindowSubstring, pySlidingWindowState

Minimum Window Substring (LeetCode 76) is where two pointers meets hash map state. Given a string s and a pattern t, find the smallest substring of s that contains all characters of t. This is a sliding window problem using same-direction two pointers, but the window state is a frequency map that tracks whether the current window satisfies the constraint. This is a hard problem, and it appears at senior levels because it tests your ability to manage complex state while maintaining the pointer logic. The approach: expand the right pointer to include characters until the window contains all required characters. Then shrink the left pointer to find the minimum valid window. Track two things: a frequency map of required characters, and a counter of how many required characters are satisfied. W