Given a binary tree, find its maximum depth.
The maximum depth of a binary tree is the number of nodes along the longest path from the root node down to the farthest leaf node.
NOTE : The path has to end on a leaf node.
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max depth = 2.
NOTE: You only need to implement the given function. Do not read input, instead use the arguments to the function. Do not print the output, instead return values as specified. Still have a question? Checkout Sample Codes for more details.