# time complexity: count common values ​​in two matrices

given two sets of integers A and B of size m, with values ​​in the
range (-n, n). I want an algorithm to count how many common values ​​are
in A and B, if a value is repeated we only count it once, to
example: $$A = {2,2,14,3 }$$ Y $$B = {1,2,14,14,5 }$$ the algorithm
I should go back 2. The problem is that I need to do this in $$O (m)$$ hour.

My attempt was to create a matrix $$C$$of size $$2n$$.
and increase all values ​​of $$A$$ Y $$B$$ by $$n$$and count the values ​​of $$A$$ I like it:
$$C (A (i)) = 1$$
that would take me $$O (m)$$ time and $$O (1)$$ Time to create the matrix.
then going $$B$$ and counting how many $$1 & # 39; s$$ I am in $$C$$.

So far it sounds good, however, I have no idea what there is $$C$$ first and it could be that there is a $$1$$ there already and that would increase the counter falsely, and initializing $$C$$ it would take $$O (n)$$ hour.

Any ideas?