Trees & Recursion
Tree DFS · Preorder
When you see it
Signal words
If the prompt uses any of these, this pattern should come to mind first.
preordercopy treecloneserializeprefixroot firstpaths from root
Practice
Drill this pattern with an AI trainer — reply only with Hint, Next, Lost, or paste your code.
Ask yourself
- Act on a node before you descend — copy, clone, serialize?
- Building a path from the root downward?
- Recurse in order: visit, then left, then right.
Classic problem
Binary Tree Preorder Traversal — Visit root before children: root → left → right. Used when you need to process a node before its subtree (e.g., serialization, path problems).
Input: Binary tree with nodes A(root), B(left), C(right), D(B-left), E(B-right), F(C-left), G(C-right)
Output: [A, B, D, E, C, F, G] — root visited first at every subtree.
Common pitfalls
Forgetting the null base case.
Checking the sum at a null child instead of a true leaf (double-counts paths).