Minimum Window Substring 30 XP Hard

Given strings s and t, return the minimum window substring in s that contains all characters of t (including duplicates).

This is the classic variable-size sliding window hard problem:

• Expand right to include more characters
• Once all chars of t are included, try shrinking from left
• Track the minimum valid window

Use a counter for needed characters and a have/need comparison. Time: O(n).