Number of distinct elements

You are given an array of $n$ numbers. Find how many distinct numbers it contains.

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

A single integer — the number of distinct values.

A set stores every value only once: adding the same value again changes nothing. Put all numbers into a set — the answer is its size. Python — set, C++ — set / unordered_set, Go — map[int64]bool.

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.