Path Sum

Return true when some path that starts at root and ends at a leaf has node values adding up to exactly targetSum. The path must run all the way down: stopping at an inner node does not count, which is what makes this a root-to-leaf check.

A leaf is a node with no children.

We use Pre-order DFS (Top-Down) logic.

Instead of calculating the sum from scratch at every leaf, we subtract the current node's value from the targetSum as we go down.

1. Consume Value: remaining = target - node.val.
2. Check Leaf: If node is a leaf (no left, no right) AND remaining == 0, we found a path! Return true.
3. Recurse: Check dfs(node.left, remaining) OR dfs(node.right, remaining). If either returns true, propagate it up.
4. Base Case: If node is null, return false.

Each node subtracts its value from what is still needed, and the remainder appears under it. A leaf that reaches exactly 0 turns green. Once a path is found, || stops, so the right side of that node is never called.