You learned how to measure trees. Diameter is the longest path between any two nodes. You can find it with two-BFS (simple) or DP (integrates with other tree computations).
The center of a tree minimizes maximum distance to all nodes. It is the middle of the diameter path. Radius is half the diameter (rounded up).
For distance queries, you used diameter endpoints (Tree Distances I) and rerooting DP (Tree Distances II). Both techniques turn problems into .
These patterns show up in network analysis, tree improvement, and competitive programming. You now have the tools to handle them.