Binary trees: each node has at most two children. Traversals: Preorder (root-left-right), Inorder (left-root-right), Postorder (left-right-root).
All . Key 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.