Insert(i, x) [1, 2, 3, 4] ---> [1, 2, 239, 3, 4]
Delete(i)    [1, 2, 239, 3, 4] ---> [1, 2, 3, 4]
implicitkey : 14A

[------] [----] [---] [x] [--][i][--] [--------]
size = sqrtN

build:
[----------------------------------------------]

Insert +2
Delete +2

if k >= 3 * sqrtN = sqrtN + 2*sqrtN
ReBuild

sqrt-decomposition
корневая оптимизация