According to this case :
Input: "a", "aa" Output: "a" Expected: ""
String T could contains duplicated characters, and the result shoud contain all of them duplicately, but the description doesn't imply it at all.
Just adding another case to the description may save a lot of time.
I saw the first question about the problem is about this, ant there is an answer, but I think we don't check the discuss until we found something wrong, so make the description more clear please if it doesn't bother a lot ~ thank you ~~~~