operator[] inserts a default-constructed value (here 0) if the key is missing, then returns a reference to it. That makes assignment as simple as m[key] = value, but it also means cout << m["missing"] will INSERT an empty entry as a side effect. For pure look-ups always use find or contains (next accordion) to avoid that surprise. insert({k, v}) only adds when the key is new and returns a pair<iterator, bool> you can inspect to see whether the insert happened.

find(key) returns an iterator pointing at the entry or end() if missing. The classic idiom is auto it = m.find(k); if (it != m.end()) { use it->second; }. Since C++20, m.contains(k) reads more clearly, but the GCC 9.2 / C++17 compiler in the test runner does not have it; count(key) is the portable fallback that returns 0 or 1. Never use m[key] for membership testing because it inserts a default value as a side effect, silently growing the map.

The classic iterator form it->first / it->second works on every C++ standard; C++17 added the cleaner for (const auto& [k, v] : map) structured-binding form, which most modern code uses. Iteration order in unordered_map is implementation-defined and may change after rehashes, so do not rely on it; use std::map if you need sorted keys. When erasing during iteration, always reassign from erase() because the old iterator becomes invalid; the C++ standard makes erase return the iterator to the next element specifically to support this pattern.