Minimum Window Substring with Repeated Required Characters

Implement `minimum_window(s, t)` to return the shortest contiguous substring of `s` containing every character in `t` with at least its required multiplicity. Character matching is case-sensitive. Return the empty string if no such substring exists. Both strings contain only uppercase and lowercase English letters and have lengths from 1 through 100,000. Inputs with a valid minimum window have a unique answer; no tie-breaking rule needs to be invented. ### Examples ```text minimum_window("ADOBECODEBANC", "ABC") -> "BANC" minimum_window("a", "aa") -> "" ``` The first result includes all three required characters. The second input cannot supply the two occurrences of `a` required by `t`. Repeated target characters must be counted; checking only distinct-character membership is insufficient. Aim for time linear in the combined string lengths. Problem reference: [LeetCode 76](https://leetcode.com/problems/minimum-window-substring/).