# Problem

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
/ \
9  20
/  \
15   7

return its depth = 3.

# Solution: Recursion

maxDepth(root) = max(maxDepth(root.left), maxDepth(root.right)) + 1

Time complexity: O(n)

Space complexity: O(n)

## Python3

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