It is possible to deamortize the O(n) resizing step, meaning you spend O(1) extra time over the O(n) operations leading to the resize, so that when the resize needs to happen, you don't need to spend O(n) time. This assumes that allocation/deallocation takes O(1) time regardless of the buffer size.

Deamortized resizing can work as follows: When inserting into a vector that is half full, allocate a new buffer twice the size and copy two elements into the buffer. For the subsequent insertions, copy two elements every time you insert one element. This guarantees that you are done copying all elements into the new buffer once the old buffer is full, and you only performed a constant number of instructions with each operation.

It's possible to deamortize many quite advanced data structures in theory - see e.g. the lecture notes on "Maintaining Order in a List" on this course webpage: https://cs.au.dk/~gerth/aa11/

... but as with everything in theoretical computer science you are not guaranteed that the implementation works better in practice than the "naive" approach. Deamortization techniques do have a lot of practical relevance in hard realtime applications, but I don't know how much of "real" hard realtime stuff uses academic techniques and how much of it is mostly engineering.

Amortized O(1)