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).
Input: 'ADOBECODEBANC', 'ABC'
Output: 'BANC'- If there is no such window in S that covers all characters in T, return the empty string "".
- If there are multiple such windows, you are guaranteed that there will always be only one unique minimum window in S.