You are given a $2D$ binary grid of size $m × n$. In this grid, $0$ represents water and $1$ represents land. At the beginning, every cell in the grid contains water (i.e., all cells are $0$).

You can perform an add land operation that changes a water cell into land. An array positions is provided, where each element positions[i] = [ri, ci] indicates the cell (ri, ci), where the ith operation is applied.

Your task is to return an array of integers answer, where answer[i] gives the number of islands present in the grid after executing the ith operation.

An island is defined as a cluster of land cells connected horizontally or vertically (diagonal connections are not considered). The grid is assumed to be completely enclosed by water along its four boundaries.

Constraints:

• $1 <=$ m, n, positions.length $<= 10^4$

• $1 <=$ m * n $<= 10^4$

• positions[i].length $== 2$

• $0 <=$ ri $<$ m

• $0 <=$ ci $<$ n