使用O（1）运行时从向量中删除元素

Question

"Write a function which takes as an input an object of vector type removes an element at the rank k in the constant time, O(1) [constant]. Assume that the order of elements does not matter."

I thought I might have had an idea about this. But, as I started to try by using .erase(), I looked up what the big-O notation was and found out it was O(n),as in linear relation. I can't think of any other way at the moment. I don't want any code, but I think pseudo code will at least point me in the right direction if anyone can help

Answer 1

Assume that the order of elements does not matter.

This is what you need to pay attention to.

Suppose you have a vector

0 1 2 3 4 5 6

and you want to remove the 3. You can turn this into

0 1 2 6 4 5

in O(1) without any issues.

Answer 2

Actually, there is a way to do it. Here is the pseudocode:

1. If the element you are trying to remove is the last element in the vector, remove it, done.
2. Read the last element of the vector and write it over the element-to-be-removed.
3. Remove the last element of the vector.

Answer 3

You can swap and pop_back in constant time.

std::swap(vec.back(), vec[rank]);
vec.pop_back();