std::set 与 std::multiset | STL
本章覆盖：set stores key；unique key；multiset duplicate key；insert；find；erase；count；lower_bound；upper_bound；equal_range；ordered traversal；key element effectively const through iterator。
本章知识点
- set stores key
- unique key
- multiset duplicate key
- insert
- find
- erase
- count
- lower_bound
- upper_bound
- equal_range
- ordered traversal
- key element effectively const through iterator