Minimum Window Substring

LC 76 — find the shortest substring of `s` that contains every character of `t` (counts respected). Asked as the second phone-screen problem in a recent SWE loop; missing the optimal-window approach gets the candidate dropped. Requirements Inputs: strings s and t. Return the minimum-length substring of s that contains every character of t (including duplicate counts). If none exists, return "". Notes Sliding-window with two hashmaps (need for t, have for the current window) and a matched counter that tracks how many distinct characters meet their needed count. Expand right, contract left whenever matched == len(need). Common slip: incrementing matched only when have[c] == need[c], not every time have[c] changes — otherwise the window won't contract correctly. Phone-screen graders explicitly look for the O(|s| + |t|) solution; the brute-force O(|s|² · |t|) version is a fail signal. Preparation Practice writing the window contraction loop without bugs: it's where most candidates blow time. Memorize the matched-counter pattern; it generalizes to LC 438 (Find All Anagrams) and LC 567 (Permutation in String).