Binary trees: each node has at most two children. Traversals: Preorder (root-left-right), Inorder (left-root-right), Postorder (left-right-root).
All . Patterns:
- Recursion with base case (null node)
- Pass information down (target sum) or up (height)
- Build trees from traversal pairs
Common Problems: Max depth, path sum, LCA, serialization. Time: . Space: where is height.