Remove duplicates

You are given an array of $n$ numbers. Keep only the first occurrence of every number and remove the other repetitions. The order of the remaining elements stays the same.

The first line contains $n$ ($1 \le n \le 10^5$). The second line contains $n$ integers ($|a_i| \le 10^9$).

The first line contains the number of remaining elements $m$. The second line contains those $m$ numbers.

Walk along the array keeping a set of "already seen" values: if a number is not in the set, append it to the answer and add it to the set; otherwise skip it. In the first example: 3 5 2.

For $n = 10^5$ comparing every element with all others ($n^2 = 10^{10}$ operations) is too slow — a set or a dictionary does it in one pass.