Maximum depth of binary tree

Time: O(N); Space: O(H); easy

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 1:

Input: root = {TreeNode} [3,9,20,None,None,15,7]

  3
 / \
9  20
  /  \
 15   7

Output: 3

[1]:

class TreeNode(object):
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution1(object):
    """
    Time: O(N)
    Space: O(H), H is height of Binary Tree
    """
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: integer
        """
        if root is None:
            return 0
        else:
            return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1

[3]:

s = Solution1()

root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20)
root.right.left = TreeNode(15)
root.right.right = TreeNode(7)
assert s.maxDepth(root) == 3