[jstimpfle]

> Trees, graphs?

Easily represented as a vertices-array and an edges-array. It's conventional to index the (directed) edges to optimize iteration over all edges from a given vertice. If you're being "sloppy", you can also represent edges as vector<vector<int>> (one array of destinations per source). This is more convenient but comes with the slight disadvantage that these arrays are physically separated (for example, you'll need two nested loops instead of only one to iterate over all edges).