I'll give you a problem: read numbers, remove duplicates, and output them sorted. Use sort, unique, and erase in combination to solve this in just a few lines. You'll write: sort(v.begin(), v.end()); v.erase(unique(v.begin(), v.end()), v.end()); for (int x : v) cout << x << " "; to produce the sorted unique output.
This pattern appears in many problems requiring deduplication. Practice variants: median finding, mode calculation, range queries. Learn the sort-unique-erase pattern.