Minimum Window Substring

Given two strings s and t, return the minimum window substring of s such that every character in t (including duplicates) is included in the window. If no such substring exists, return "".

The answer is guaranteed to be unique.

Example 1:

Input:  s = "ADOBECODEBANC", t = "ABC"
Output: "BANC"

Example 2:

Input:  s = "a", t = "a"
Output: "a"

Example 3:

Input:  s = "a", t = "aa"
Output: ""
Explanation: t requires two 'a's, but s only has one.

Constraints:

• m == s.length, n == t.length
• 1 <= m, n <= 10⁵
• s and t consist of uppercase and lowercase English letters

Follow-up: O(m + n) algorithm?