You have gone deep into trees. - Structure: Root, Parent, Child, Leaf. - Traversal: DFS with parent check (no visited array). - Tree DP: Calculating sizes/heights bottom-up. - Diameter: The longest path (Double BFS). - Centers: Peeling the tree layer by layer.
Trees are effective, but they are static. In the next section, you look at how to manage groups that change over time using Disjoint Set Union (DSU).