Given two strings s and t, find the minimum window substring of s that contains all characters of t (including duplicates). Return an empty string if no such window exists.
With s = "ADOBECODEBANC" and t = "ABC", the answer is "BANC". This is the smallest substring of s that contains 'A', 'B', and 'C'.
Note that t may have duplicate characters. If t = "AA", your window must contain at least two 'A's.
Constraints: .