Problem Statement
Determine the depth (or height) of a binary tree, which refers to the number of nodes along the longest path from the root node to the farthest leaf node. If the tree is empty, the depth is 0.
Example:

 Input
1 / \ 2 3 / \ 4 5
Output: 3
Explanation: The longest path is1>2>4
or1>2>5
with 3 nodes. 
 Input:
1 \ 2 \ 3
 Expected Output: 3
 Justification: There's only one path
1>2>3
with 3 nodes.
 Input:

 Input:
1 / \ 2 3 / \ 4 7 \ 9
 Expected Output: 4
 Justification: The longest path is
1>2>7>9
with 4 nodes.
 Input:
Constraints:
 The number of nodes in the tree is in the range [0, 10<sup>4</sup>].
100 <= Node.val <= 100
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Python3
Python3