https://leetcode.com/problems/minimum-window-substring/
Hard
Given a string S and a string T, find the minimum window in S which will contain all the characters in T in complexity O(n).
Example:
Input: S = "ADOBECODEBANC", T = "ABC"
Output: "BANC"
Note:
- If there is no such window in S that covers all characters in T, return the empty string "".
- If there is such window, you are guaranteed that there will always be only one unique minimum window in S.