两个键之间的键数

Question

如何在地图中找到两个键值之间出现的键数？有没有办法用迭代器来完成这个？

Answer 1

看看地图和一些STL algirhtims：

http://www.cplusplus.com/reference/stl/map/
    lower_bound Return iterator to lower bound (public member function)
    upper_bound Return iterator to upper bound (public member function) 
    distance Calculates the number of elements between first and last.