This is the link of problem: D. White Line.
For the best i understand up to now, let's focus on row.
I will base on the range [l;r] each row of black to determine whether which cells that we click will erase this row or not. These cells are in the rectangle from $$$(l1, l2)$$$ (top left) to $$$(r1-1, r2-1)$$$ (bottom right), $$$l1, l2, r1, r2$$$ is defined in the code i extracted from @kcm1770's solution.

         for(int i=0; i<n; ++i) {
		int l=0;
		while(l<n&&s[i][l]=='W')
			++l;
		if(l>=n) {
			++b[0][0];
			continue;
		}
		int r=n-1;
		while(~r&&s[i][r]=='W')
			--r;
		if(r-l+1>k)
			continue;
		//i-k+1, i
		//r-k+1, l
		int l1=max(i-k+1, 0), r1=i+1;
		int l2=max(r-k+1, 0), r2=l+1;
		++b[l1][l2];
		--b[l1][r2];
		--b[r1][l2];
		++b[r1][r2];
	}

I see a lot of people do the same approach. I don't know how they come up with the way set ++ & -- like that. This is all of magic for me. Could you tell me about some similar problems using that or some concepts involve.