DFS Recursive Patterns

Concepts: pyDFSMaxDepth, pyDFSPathSum, pyDFSSymmetric

Beyond 'return all values in order,' most tree interview problems use DFS to compute something about the tree structure: max depth, whether a path sum exists, whether the tree is symmetric. The mental model shift from 'traverse and collect' to 'return a value from each subtree and combine' is the most important thing you can internalize from this section. When you get it, hard tree problems become medium. When you don't, medium tree problems feel impossible. Pattern 1: Max Depth Pattern 2: Path Sum Pattern 3: Symmetric Tree The three problems above (max depth, path sum, symmetric tree) are among the most common phone screen warm-ups. If you can solve all three fluently before the interviewer finishes their coffee, you set a strong tone for the rest of the interview. Practice until each tak