Resizable arrays with fast insertion/deletion

• O(1) constant time for random access arr.(i) and updates arr.(i) <- v
• O(1) amortized for push_front and pop_front, push_back and pop_back to add or remove an element to the start or the end
• O(sqrt N) for insert_at arr i x and delete_at arr i to insert or delete an element anywhere else

This datastructure was invented by Goodrich and Kloss and is described in their paper "Tiered Vectors: Efficient Dynamic Arrays for Rank-Based Sequences".