Maximum Depth (or Height) of Binary Tree (easy)
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:
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Python3
Python3
. . .