Mention the complexity of an operation

[richelbilderbeek]

trafficstars

Here is the thread:

https://www.reddit.com/r/cpp/comments/3xw5q0/a_wellconnected_c11_boostgraph_tutorial/

From mat69:

• You should also mention the complexity of operations. The way you retrieve a vertex by name is O(n). I guess in most use cases it will be more efficient to have an std::unordered_map<std::string, MyVertex> and doing a lookup instead.