index array might be not in ascending order

[AndrewLisuxin]

LeetCode-Solutions/C++/insert-delete-getrandom-o1-duplicates-allowed.cpp

Line 27 in b7cfbdf

used_[list_.back()].back() = used_[val].back();

seems you think the tail element in the storage is corresponding to the tail element in the index array, which
might be wrong after you do some swap

[kamyu104]

I fixed it. Thank you very much!