D&C applies to trees via centroid decomposition.
Centroid: A node whose removal splits the tree into subtrees of size .
Approach: Find the centroid.
Solve the problem for paths through the centroid.
Remove the centroid, recurse on each subtree.
Why it works: The centroid exists for any tree. After levels of decomposition, all nodes are processed.
Applications: Counting paths with specific properties, tree distance queries.
This is an advanced technique covered in the Graph Theory roadmap. The core idea is that D&C works on any structure where you can split into balanced parts.