Minimum Window Substring

## Problem Given two strings `s` and `t` of lengths `m` and `n` respectively, return the **minimum window substring** of `s` such that every character in `t` (including duplicates) is included in the window. If there is no such substring, return the empty string `""`. The testcases will be generated such that the answer is **unique**. ## Examples **Example 1:** ``` Input: s = "ADOBECODEBANC", t = "ABC" Output: "BANC" Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t. ``` **Example 2:** ``` Input: s = "a", t = "a" Output: "a" Explanation: The entire string s is the minimum window. ``` **Example 3:** ``` Input: s = "a", t = "aa" Output: "" Explanation: Both 'a's from t must be included in the window. Since the largest window of s only has one 'a', return empty string. ``` ## Approach Hints Use a sliding window with two pointers. Expand the right pointer to include all characters of `t`, then contract the left pointer to minimize the window. Use frequency maps to track character counts.