Binary Trees

Not started yet — 18 problems queued, opening Trees once Binary Search and Recursion are solid.

DFS (pre/in/post-order, recursive or iterative with an explicit stack) explores depth-first; BFS (level-order, queue-based) explores breadth-first. Most tree problems reduce to "what do I compute at this node from my children's results" — think bottom-up (post-order) vs. top-down (passing state downward).

Sub-patterns

A — DFS Traversals. Pre/in/post-order, recursive or with an explicit stack.
B — BFS / Level-Order. Queue-driven, level by level.
C — Path Problems. Root-to-leaf and any-to-any path sums and lengths.
D — Tree Construction from Traversals. Rebuild a tree given two traversal orders.
E — Symmetry & Comparison. Comparing a tree against itself or another tree.
F — Lowest Common Ancestor. Find the deepest shared ancestor of two nodes.

Practice set (18)

Easy — Maximum Depth of Binary Tree, Invert Binary Tree, Same Tree, Symmetric Tree, Path Sum, Diameter of Binary Tree, Balanced Binary Tree
Medium — Binary Tree Level Order Traversal, Binary Tree Zigzag Level Order Traversal, Construct Binary Tree from Preorder and Inorder Traversal, Lowest Common Ancestor of a Binary Tree, Path Sum II, Binary Tree Right Side View, Flatten Binary Tree to Linked List, Populating Next Right Pointers in Each Node
Hard — Binary Tree Maximum Path Sum, Serialize and Deserialize Binary Tree, Vertical Order Traversal of a Binary Tree

Pending