Write a method to find the minimum sequence in a seq string,
such that the sequence contains all the characters in a word string.
Try and limit the runtime complexity to O(n).
If no sequence is found, return an empty string.
Examples:
source = "TALMEBECCOFDEPATCB"
word = "ABC"
smallestWindow(source,word) ==> "ATCB"
source = "GBNRATE"
word = "ABC"
smallestWindow(source,word) ==> "[Empty]"
