does not need to start or end at the root or a leaf, but it must go downward (travelling only from parent nodes to child nodes). 1. Carry a running sum along the DFS path. 2. Hash map tells us how ...

the tree has a root-to-leaf path whose node values sum to exactly `targetSum`. A leaf is a node with no children. An empty tree has no such path. 1. Base case: None node → return False (no path ends ...