root of a binary tree. A leaf is a node with no children.
Find the shortest path from the root down to any leaf and return the number of nodes on it, counting both the root and the leaf. If the tree is empty, return 0.root=tree_node([6, 2, 8, None, None, 7, 9]) → 2
Node 2 is a leaf right under the root, and the path 6 → 2 has two nodes.
root=tree_node([1, None, 4, 3]) → 3
The root has no left child but is not a leaf itself; the only leaf is 3, reached by 1 → 4 → 3.
root=tree_node([]) → 0
An empty tree has no nodes, so the answer is 0.
Node.val ≤ 1000root=tree_node([6, 2, 8, None, None, 7, 9])
2
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