Section Recap

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

All $O(n)$. 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: $O(n)$. Space: $O(h)$ where $h$ is height.