Exercise: remove duplicates

Write a function std::vector<int> dedupe(const std::vector<int>& v). It returns a new vector with the values of v in the same order, but each one only at its first occurrence.

• dedupe({3, 1, 3, 2, 1}) → {3, 1, 2}
• dedupe({5, 5, 5}) → {5}
• dedupe({}) → {}

Constraints: v may be empty, and its values are any int numbers, negative ones included.