Minimum Window Substring

Write `min_window(s, t)` returning the shortest substring of `s` containing every character of `t` including duplicates. Return `""` when no such window exists. min_window("ADOBECODEBANC", "ABC") -> "BANC" Grow the right edge until the window is valid, then shrink the left edge while it stays valid, recording the best. The only way to keep this O(n) is a single counter of how many required characters are currently satisfied — checking validity by rescanning the window makes it O(n * k).