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).
S = "ADOBECODEBANC" T = "ABC" #Minimum window is "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.
You may assume all inputs are valid and each input a window exists.